28 What to Draw in a Sketchbook on a Rainy Day in the City

Rainy days are made for working out what to draw in a sketchbook, because the city slows down, the colours go soft and everyone under an umbrella becomes a tiny story. You do not need sunshine or a tidy composition plan, just a window, a doorway or a puddle and a pen that does not mind a few wobbles.

Below are 28 small scenes built for exactly this: a fineliner line, a quick wash of watercolor or marker, and a page that reads as warm and lived in rather than careful. Work through them in any order, start with whatever is outside your own window today, and let the ink run a little where the rain does.

Start the page with a girl with a red umbrella crossing a zebra crossing, because the strong red dome against grey asphalt is an easy win and it anchors the whole sketchbook spread. Draw the umbrella first as a simple curved triangle, then drop the figure in underneath it as a few soft body shapes, legs slightly bent mid stride.

Ink the umbrella ribs and the crossing stripes with the fineliner before any color goes down, keeping the stripes loose rather than ruler straight. Wash the umbrella in a flat red, let it dry, then add the wet road in grey watercolor with the stripes left as the white paper showing through.

If the proportions feel off, measure the umbrella against the figure first: the umbrella canopy should sit about as wide as the girl is tall, which keeps her from looking swallowed by it. If you do not have red watercolor, a red marker on top of dry ink works just as well.

Finish with one dark puddle shadow under her boots to ground the whole scene.

Palette: Cadmium Red,Payne’s Grey,Ivory Black,Cool Grey

Follow the umbrella with a quieter scene: a steamed up cafe window with a reader inside, seen from the wet street looking in. Pencil a simple window frame first, then draw the reader as a soft rounded shape hunched over a book or cup, kept small and loose since she sits behind glass.

Ink the window frame and the reader’s outline, then leave patches of the glass undrawn to suggest the steam. Lay a warm yellow wash inside the window for the cafe light, and a cool grey wash outside for the wet street, letting the two colors meet softly at the window edge.

Eye level matters here: keep the window at your own standing height, roughly centered on the page, so the light inside reads as glowing rather than floating. If marker bleeds too fast on your paper, switch the warm glow to a dry brush of yellow ochre instead.

A few short diagonal lines outside the glass finish the rain without any extra fuss.

Palette: Yellow Ochre,Payne’s Grey,Burnt Sienna,Ivory Black

Look at real cafe windows at dusk for reference: the warm light almost always sits at the center of the glass, fading to cooler tones near the frame edges.

Give the page some movement with a dog in a yellow raincoat trotting along a wet pavement, lead trailing behind. Block the dog in as three ovals (head, chest, hindquarters) in pencil first, then draw the little raincoat over the chest and back like a simple saddle shape.

Ink the dog’s outline and the coat’s edges, keeping the legs short strokes rather than stiff lines so the trot feels springy. Wash the coat in bright yellow, the fur in a warm grey or tan, and let a thin blue grey shadow pool under the paws.

Dogs go wrong fastest at the leg joints, so if this is your first time drawing one, sketch the four legs as simple bent lines before you commit to ink, checking that the front legs are shorter than the body is long. No yellow marker on hand works fine too: a yellow colored pencil layered under a light wash gives the same cheerful coat.

One bright drip of water off the hem of the coat sells the whole rain story.

Palette: Cadmium Yellow,Raw Umber,Payne’s Grey,Ivory Black

After a moving subject, slow the page down with puddles reflecting street lamps along an empty wet street at dusk. Draw the lamp posts first as simple vertical lines with a small round or teardrop lamp head, spaced unevenly so the street feels real rather than ruled.

Ink the lamp posts and a loose horizon line for the pavement edge, then paint the puddles as flattened ovals directly below each lamp. Drop a streak of warm yellow or orange straight down into each puddle shape while the blue grey street wash is still wet, so the color blooms and mirrors the lamp above it.

This is the dry brush trick worth knowing: once the puddle wash is dry, drag a nearly dry brush loaded with dark grey across the surface in one quick stroke to suggest a ripple without redrawing the whole reflection. If you do not own a fine brush, the edge of an old credit card dragged through wet paint gives a similar streak.

Keep the sky above plain and dark so the lamps read as the only light.

Palette: Indigo,Cadmium Orange,Payne’s Grey,Ivory Black

Bring people back in with a bus stop with strangers under one umbrella, a small comedy of elbows and shoulders squeezed together. Sketch the umbrella canopy first as one wide dome, then tuck three simple standing figures underneath it at slightly different heights so none of them overlap awkwardly.

Ink the umbrella and the figures, keeping faces as the barest suggestion of a nose and chin since this scene is about the group, not the individuals. Wash each coat a different flat color so the three people stay easy to tell apart, and leave the bus stop shelter behind them as a loose grey rectangle.

Ten minutes gets you the umbrella and the three figures in flat color and nothing else, which already tells the joke. Give it forty minutes and add a bus shelter sign, a puddle at their feet and a bit of texture in the coats, and the whole page feels like a finished scene rather than a sketch.

One tip for today: stagger the figures’ heads so no two sit on the same horizontal line.

Palette: Prussian Blue,Burnt Sienna,Sap Green,Payne’s Grey

Give the page some color with a flower stall covered in plastic sheets, buckets of blooms glowing through the clear covers. Draw the stall frame first as a simple box with a slanted roof, then fill it with loose bucket shapes holding a few larger flower heads rather than dozens of tiny ones.

Ink the stall structure and the flower outlines before the plastic goes on top, then lay the plastic sheeting over everything last as a few faint diagonal lines and a pale grey wash with soft white highlights left unpainted where the sheet catches the light.

Flowers at this scale go wrong when they turn into scribble, so keep each bloom to three or four simple petal shapes and let color do the rest. For an easier swap, draw tulips instead of any fussier flower, since their closed cup shape is the fastest flower on any sketchbook page.

Finish with one bright color note (a single red bloom) to pull the eye into the stall first.

Palette: Permanent Rose,Sap Green,Cadmium Yellow,Cool Grey

Move the viewpoint inside for raindrops on a tram window, looking out at a blurred city street through streaked glass. Pencil a soft rectangle for the window frame first, then draw the outside world beyond it as loose, simplified shapes, since the glass should blur everything behind it.

Ink only a handful of individual raindrops as small circles and short trailing lines down the glass, leaving most of the window undrawn. Wash the outside scene in muted colors first, let it dry completely, then paint the raindrops themselves as tiny clear highlights with a touch of white gouache or gel pen over the dry wash.

The eye naturally sits level with the window on a tram, so keep the outside shapes low and horizontal rather than looking up, which keeps the perspective honest. If you do not have white gouache, a white gel pen does the same highlight job straight onto dry watercolor.

Leave the glass mostly open paper: raindrops read better with space around them than crowded together.

You Will Need

• white gel pen

Palette: Payne’s Grey,Prussian Blue,Ivory Black,Titanium White

Take a breath with a quieter subject: a cat watching the rain from a shop shelf, tucked among jars or books behind the glass. Draw the cat first as a simple rounded loaf shape with ears and a tail curled around its paws, since a sitting cat is mostly soft curves rather than sharp lines.

Ink the cat’s outline and a few whisker lines, then sketch the shelf and a couple of simple shop objects beside it, keeping them loose so the cat stays the star. Wash the cat in a warm tabby orange or a flat grey, and let the shop interior behind stay dim and understated.

If the cat’s proportions wander, compare the head width to the body: a seated cat’s head is usually about a third of its total height, which keeps it from looking too large or too small. No orange paint handy means a warm brown marker gives the same cosy tabby feel.

Add two round, slightly sleepy eyes last, since cats read as content the moment the eyes relax.

Palette: Burnt Sienna,Raw Umber,Payne’s Grey,Ivory Black

Pick the energy back up with a cyclist in a bright poncho, pedaling through the wet with the fabric billowing behind. Draw the bicycle frame first as a simple triangle and two wheel circles, then add the rider on top, poncho drawn as one loose draped shape that hides most of the arms and shoulders.

Ink the bicycle and figure before the poncho color, then wash the poncho in one bold flat color that catches the eye against the grey street, letting a few fabric folds show as slightly darker lines once the first wash is dry.

Bicycles wobble easily on the page, so check that both wheels sit on the same ground line before you ink the frame. If bright marker is not in your kit, a bold watercolor like pink or orange gives the poncho just as much punch.

Finish with a short motion line or two trailing behind the back wheel to carry the speed.

Palette: Cadmium Orange,Payne’s Grey,Ivory Black,Cool Grey

Settle into a street scene with wet cobblestones outside a glowing bakery, warm light spilling onto the stones from the window. Draw the bakery facade first as a simple building front with one large window, then lay the cobblestones below it as loose, uneven rounded shapes rather than a tidy grid.

Ink the building and a few pastry shapes visible in the window, then wash the window interior in warm yellow and the cobblestones in cool blue grey, letting a streak of the window’s yellow bleed down into the stones directly below it while the wash is still damp.

Cobblestones go stiff if drawn one by one in neat rows, so vary their size and spacing and let some disappear into shadow near the edges of the page. For an easier version with the same warmth, a flower shop window works just as well as the light source if a bakery feels fussy.

One tip: paint the glow before the stones, so the color has somewhere warm to bleed into.

Palette: Cadmium Yellow,Payne’s Grey,Burnt Sienna,Prussian Blue

Bring the romance in with a couple sharing one coat, held over both their heads against the rain as they walk close together. Sketch the two figures first as simple leaning shapes angled slightly toward each other, then draw the coat stretched between them like a small tent, one arm from each person holding it up.

Ink the two figures and the coat shape, keeping faces turned slightly away or down so the pose carries the feeling rather than expressions. Wash the coat in one warm color and the two outfits underneath in contrasting cooler tones, then add a shared puddle shadow at their joined feet.

Eye level stays at standing height and slightly behind the pair, which keeps the coat reading as sheltering them rather than floating above. No warm brown on hand works fine with a deep red coat instead, since the contrast against grey rain matters more than the exact shade.

Angle both figures’ feet the same direction so the walk feels like one movement, not two people standing still.

Palette: Burnt Sienna,Prussian Blue,Payne’s Grey,Ivory Black

Lift the energy again with kids jumping in puddles in rain boots, caught mid air with water splashing up around their boots. Draw the boots first, since the bright colored rain boots are the whole joke of this scene, then build the rest of each small body upward from there in simple rounded shapes.

Ink the figures and boots, then add the splash as a few quick upward flicks of white gouache or a wet brush dragged outward from the puddle’s edge once the blue grey puddle wash is dry.

Quick version: one child, one puddle, ten minutes, flat colors only. The forty minute version adds a second child, a parent’s legs at the edge of the frame, and more spray in the splash, which turns a single jump into a whole moment.

If this is your first time drawing a jump pose, bend both knees slightly and tilt the torso forward, since a straight stiff body never looks like it left the ground.

You Will Need

• white gouache or white gel pen

Palette: Cadmium Yellow,Cadmium Red,Payne’s Grey,Titanium White

Slow the pace with a bookshop doorway under a dripping awning, books stacked in the window just inside. Draw the doorway and awning frame first, angling the awning slightly forward so it reads as sheltering the step below, then sketch a loose pile of book shapes in the window rather than individual spines.

Ink the doorway, awning edge and window display, then wash the awning in a muted stripe pattern and the window glow in warm amber, letting a thin dark line of shadow sit just under the awning’s edge where the drips fall.

Drips are the detail that sells this scene, so add them last as a few short vertical flicks of the pen along the awning’s lowest edge, spaced unevenly rather than in a neat row. If amber feels too orange for your palette, a warm yellow ochre wash gives the same cosy glow.

Keep the doorway dark and recessed so the eye reads it as deep rather than flat against the building front.

Palette: Yellow Ochre,Burnt Umber,Payne’s Grey,Ivory Black

If you enjoy this kind of cosy shopfront scene, the watercolor city ideas collection has more rainy street corners worth sketching.

Pick up the pace of the street with taxis with headlights in the drizzle, a small queue of cars glowing along a wet road. Draw the nearest taxi first as a simple rounded rectangle body with two headlight circles, then add one or two more taxis behind it, smaller and less detailed to suggest the line continuing.

Ink the taxi shapes and headlights, then wash the body color flat and bright, and paint the headlight beams as soft yellow cones that fade out into the grey wet road rather than stopping with a hard edge.

Light sources matter most here: keep every headlight beam pointing the same direction, toward the viewer or along the road, so the light feels consistent across all the cars. No yellow marker to hand works just as well with a pale yellow watercolor wash softened with a damp brush at the edges.

One finishing touch: a faint reflection of each headlight on the wet road ties the whole row together.

Palette: Cadmium Yellow,Cadmium Yellow,Payne’s Grey,Ivory Black

Bring in a touch of plant life with a balcony of geraniums in the rain, pots lined along an iron railing above the street. Draw the railing first as a simple repeating pattern of vertical bars, then add each flower pot as a rounded shape with a cluster of red blooms on top, keeping the flowers to a few bold heads rather than many tiny petals.

Ink the railing and pots before any color, then wash the geraniums in bright red and the leaves in a deep green, letting a few raindrops sit as small unpainted circles left as bare paper within the wet leaves.

If you do not have a fine brush for the railing bars, the edge of a ruler guides a straight pen line just as well. For variety next time, swap the geraniums for trailing ivy or a window box of herbs using the same pot and railing setup.

Keep the street below soft and blurred so the balcony stays the clear focus of the page.

Palette: Cadmium Red,Sap Green,Payne’s Grey,Ivory Black

Change the angle entirely with museum steps full of umbrellas seen from above, a bird’s eye cluster of colorful domes climbing a staircase. Draw the steps first as a few simple curved or straight bands receding upward, then scatter umbrella circles across them, overlapping slightly and varying in size as they climb further from the viewer.

Ink the steps and umbrella outlines, then wash each umbrella a different flat color so the cluster reads as cheerful rather than muddy, and keep the steps themselves pale grey so the umbrellas stand out clearly against them.

Looking straight down changes the whole drawing: umbrellas become simple circles rather than domes, and no figures need to show at all, since the bodies hide beneath. If a full staircase feels like too much for one sitting, four or five umbrellas on a short run of steps tells the same story.

Vary the umbrella sizes slightly even on the same step, since perfectly matched circles read as stamped rather than drawn.

Palette: Cadmium Red,Prussian Blue,Cadmium Yellow,Cool Grey

Bring the page back down to a small, quiet moment with a pigeon sheltering under a statue, tucked into the stone base out of the rain. Draw the statue base first as a simple geometric block or plinth shape, then add the pigeon beneath it as a rounded body with puffed feathers and a small tucked head.

Ink the statue edge and the pigeon’s outline, then wash the stone in cool grey and the pigeon in a soft dove grey with a touch of pink or green at the neck, the one spot of color pigeons actually have.

Pigeons read as puffed and dry if the body shape stays round and the head tucks in close to the chest, so if proportions feel tricky, compare the body width to the head: the head should be noticeably smaller, almost a third of the body’s width. No pink or green on hand is fine, since a plain grey pigeon still reads perfectly well against the stone.

Leave a narrow dry patch of paper under the statue ledge to show exactly where the shelter line falls.

Palette: Payne’s Grey,Cool Grey,Permanent Rose,Ivory Black

Climb up to a girl reading on the top deck of a bus, seen through the long row of windows as the bus moves past. Draw the row of windows first as a simple repeating rectangle pattern along the bus body, then place the reading figure inside just one of them, small and settled, book held low in her lap.

Ink the bus outline and windows before adding the figure, then wash the bus body in one bold flat color and the windows mostly dark, except for the single lit window where the girl sits, kept warm and pale so the eye goes straight there.

This is the one window that matters, so treat the rest of the bus as simple background: keep every other window plain and unlit. If bus red is not in your palette, any strong color works, since the single warm window against a darker body is what carries the scene.

A soft blur along the bottom edge of the bus suggests motion without any extra lines needed.

Palette: Cadmium Red,Payne’s Grey,Yellow Ochre,Ivory Black

Keep the street moving with a delivery bike in the rain, rider leaning forward with a large insulated box strapped to the back. Draw the bike frame and box first, since the box is the visual anchor of this scene, then add the rider leaning over the handlebars in a simple hunched pose.

Ink the bike, box and rider, then wash the delivery box in a bright flat color, since real delivery bags are bold on purpose, and keep the rider’s rain gear in a duller tone so the box still reads as the brightest shape on the page.

One hands on trick for the wet road under fast moving wheels: once the grey road wash is dry, drag a nearly dry brush horizontally behind the back wheel to suggest spray without drawing individual droplets. If a dry brush feels fiddly, a few quick pencil scribbles erased halfway with a kneaded eraser gives a softer version of the same spray.

Angle the whole bike slightly uphill on the page to add a sense of speed.

You Will Need

• kneaded eraser

Palette: Cadmium Orange,Payne’s Grey,Ivory Black,Cool Grey

Slow right down with a lit laundromat at dusk, rows of round machine windows glowing against the dark street outside. Draw the laundromat front first as a simple grid of machine shapes along the wall, each one a circle set inside a square, then add a single small figure waiting inside if the page has room.

Ink the grid of machines and the shopfront outline, then wash the interior in warm yellow light and leave the street outside in cool dusk blue, letting the warm color spill just slightly past the window edge into the street.

If you do not have a wide yellow marker for the glow, layering two thin coats of yellow watercolor gives the same steady warmth without streaking. For variety next time, try the same warm glowing grid as a bakery display case or a small corner shop front instead.

Keep every machine window a slightly different shade of warm light, since uniform color flattens the whole row.

Palette: Cadmium Yellow,Indigo,Burnt Sienna,Payne’s Grey

Come in from the street for a bowl of soup at a window counter, steam rising gently beside the rainy glass just behind it. Draw the bowl first as a simple rounded shape with a thin rim ellipse, then add the window and rain streaks behind it, kept blurry and secondary to the bowl itself.

Ink the bowl, spoon and window frame, then wash the soup in a warm orange or red and the steam as a few soft curling pencil lines left mostly unpainted, since steam reads better as open paper than as heavy color.

If you do not have much time, ten minutes gets you the bowl and window in flat color; the forty minute version adds a napkin, a second cup, and more detail in the rain streaks outside, turning a simple still life into a small cosy scene.

One tip for today: paint the steam last and sparingly, since too much white space removed kills the gentle rising feeling.

Palette: Cadmium Orange,Burnt Sienna,Payne’s Grey,Titanium White

Step into the park for a duck family crossing a wet park path, mother duck leading a small line of ducklings across the gravel. Draw the mother duck first as a simple rounded body with a slightly raised head, then line up three or four small round ducklings behind her, each slightly smaller than the last.

Ink the whole line before any color, then wash the mother in a muted brown and the ducklings in soft yellow, letting the wet path beneath them stay a cool grey with a thin reflected line of yellow under each duckling.

Ducklings crowd together if drawn too evenly, so space them with slightly uneven gaps and tilt a head or two sideways, since real ducklings never walk in a perfect row. No yellow paint handy works fine with a pale cream wash instead, since the soft color matters more than the exact shade.

Keep the path texture minimal: a few loose dashes read as gravel better than a fully rendered surface.

Palette: Raw Umber,Cadmium Yellow,Payne’s Grey,Cool Grey

For more small creatures with a bit of character, the cute animal drawing ideas list has plenty worth adding to your sketchbook.

Lift the mood with a rainbow over rooftops after the rain, a wide arc appearing above a simple city skyline. Draw the rooftop silhouettes first as a row of simple rectangle and triangle shapes along the bottom of the page, then pencil a faint guide arc above them before any ink touches the paper.

Ink only the rooftops, leaving the sky and rainbow area completely untouched, then paint the rainbow as soft overlapping bands of color, working from red at the outer edge inward, keeping each band loose and slightly blended into its neighbor rather than sharply separated.

Where this goes wrong is usually color order and a too solid, cartoonish band, so keep the washes pale and let them overlap just slightly at the edges for a believable soft arc. No full rainbow set of paints is needed either: three or four bands (red, yellow, blue) still read clearly as a rainbow from a short distance.

Finish with the rooftops in solid dark silhouette so the pale rainbow above has something strong to contrast against.

Palette: Cadmium Red,Cadmium Yellow,Cerulean Blue,Ivory Black

Bring the view back inside with a window seat with tea and rain streaks, a cup resting on a sill beside a blurred wet street below. Draw the window frame and sill first, then add the teacup and saucer as simple curved shapes sitting just inside the frame, with a thin curl of steam rising beside it.

Ink the cup, sill and window frame, then wash the street outside in muted cool tones through the glass and the tea itself in warm amber, letting the window frame stay crisp while the street beyond blurs softly at its edges.

If you would rather try this scene with a hint of perspective, tilt the sill slightly and let the street below recede at an angle instead of sitting flat, which gives the whole page more depth. A mug swapped for the cup and saucer works just as well if you prefer drawing a simpler rounded shape.

Keep the rain streaks on the glass as just a few diagonal lines, since too many flattens the whole window.

Palette: Burnt Sienna,Payne’s Grey,Yellow Ochre,Cool Grey

Add some sound to the page with a street musician under a stone arch, instrument case open at their feet for coins. Draw the stone arch first as a simple curved frame, then place the musician inside it, instrument held in a clear simple pose such as a guitar angled across the body or a violin raised to the chin.

Ink the arch and figure, then wash the stone in cool grey, the musician’s coat in one warm standout color, and the open instrument case in a dark flat tone with a few small coin dots left as bare paper or a pale dot of gel pen.

Lighting shifts the whole mood here: if the light comes from outside the arch, the musician sits mostly in shadow with a bright rim of light along one shoulder, which reads as more atmospheric than flat even lighting. If a guitar feels tricky to draw, an accordion or a simple hand drum is an easier shape for a first attempt.

Keep the stone arch darker at the top, since light rarely reaches that far into a curved doorway.

Palette: Cool Grey,Burnt Sienna,Payne’s Grey,Ivory Black

Bring the rain story indoors with rain boots drying by a front door, a small still life of two pairs set side by side on a mat. Draw the boots first as simple rounded cylinder shapes, slightly leaning against each other or the wall, then add the door and mat behind them as a plain simple backdrop.

Ink the boots and the door frame, then wash each pair a different bright color so the still life has some visual interest, and leave a thin unpainted highlight stripe down the side of each boot to suggest the shine of wet rubber.

For your next attempt at this same still life, change up the number and style of boots, mixing in a small pair beside the adults’ to suggest a whole family lives there. If bright rubber boot colors are not in your palette, a single deep green or navy pair still reads clearly against a plain mat.

One concrete tip: angle the boots slightly toward each other rather than drawing them perfectly parallel, since a small lean makes them look just kicked off rather than arranged.

Palette: Cadmium Yellow,Prussian Blue,Burnt Umber,Cool Grey

Pick up the pace again with a florist running with buckets of tulips, water sloshing slightly as she hurries through the rain. Draw the running figure first in a clear forward lean, one knee lifted, then add the two bucket shapes swinging from her hands, each filled with a loose cluster of tulip heads rather than many tiny stems.

Ink the figure and buckets, then wash the tulips in two or three bold colors across the buckets, and splash a few quick water droplets flicking up from the bucket rims using a flicked brush or a dry toothbrush loaded with pale grey paint.

Running figures look stiff if both arms swing the same way, so check that the opposite arm and leg lead together (right arm forward with left leg forward), the same rule that makes any walking or running pose convincing. No toothbrush on hand is fine, since a few flicked brush bristles give the same splatter effect.

Keep her coat flying slightly behind her to finish the sense of hurry.

You Will Need

• toothbrush or splatter brush

Palette: Permanent Rose,Cadmium Yellow,Sap Green,Payne’s Grey

If tulips and florist scenes appeal, the watercolor spring flower ideas list has more blooms worth sketching close up.

Close the sketchbook spread with a cafe terrace with striped awning in the rain, empty chairs and tables sheltered beneath it while the street beyond stays grey and wet. Draw the awning first as a simple triangular canopy with alternating stripes, then add a row of small round tables and chairs underneath it, kept loose and slightly uneven.

Ink the awning, tables and chairs, then wash the stripes in two alternating colors, the terrace floor in a warm neutral, and the street beyond in cool wet grey, letting the warm terrace colors stop clearly at the awning’s edge.

This closing scene pulls together everything the rest of the page has practiced: warm light against cool rain, a few simple shapes standing in for a whole setting. If a full striped awning feels like a lot for one sitting, two tables and a half awning across the edge of the page still reads as a complete little scene.

Finish with one small detail like a cup left on a table to suggest someone just stepped inside.

Palette: Cadmium Red,Titanium White,Payne’s Grey,Burnt Sienna

For more cosy rainy cafe scenes to round out this sketchbook page, the rainy day painting ideas collection has plenty to choose from.

What to Do Next

Pick whichever scene matches the mood outside your own window today, grab the cream sketchbook and the fineliner, and let the first few minutes just be loose pencil shapes before any ink goes down. One finished page is plenty for today, and the next rainy afternoon will bring another.

Frequently Asked Questions

What is the easiest thing to draw in a sketchbook when it is raining outside?

A single puddle reflecting a street lamp or a cat watching from a window is the easiest start, since both need only a few simple shapes and one warm color accent against grey.

Do I need watercolor to draw these rainy city scenes?

No, a fineliner alone captures every scene here perfectly well. Watercolor or brush markers add the warm glow and grey wet tones, but flat colored pencil or marker works just as well if that is what you have.

How do I stop my sketchbook page from looking too empty or too busy?

Pick one light source or one bright color accent per scene and let everything else stay simple and loose. A single warm window or umbrella against plain grey rain always reads clearer than many competing details.

What paper is best for ink and watercolor sketches like these?

A sketchbook with at least slightly heavier cream toned paper, made for mixed media or light watercolor, handles both fineliner and a thin wash without buckling or bleeding through.

How long should one of these rainy city sketches take?

Most of these scenes finish in fifteen to forty minutes depending on detail. Simple ones like a single umbrella or a cup of tea sketch quickly, while group scenes with several figures take closer to the full forty minutes.