WARNING: Town mods may lower your frames, this is the down side to town mods. Using these mods will more than likely cause a drop in frames.

This guide will give you a list of town mods that either expand, clutter or even add all new towns to Skyrim.

Mods:
jkrojmal: https://www.nexusmods.com/skyrimspecialedition/users/1305814
This is the popular choice, well at least among the nexus. JK covers all the towns and cities of Skyrim, adding new buildings and lots and lots of clutter. JKs is considered bloat to some, and with good reason, you need a decent PC to get decent frames with his mods, but all the same I suggest you look into them.

Capital Windhelm: https://www.nexusmods.com/skyrimspecialedition/mods/42990
Turns Windhelm into the capital it was always meant to be.

Capital Whiterun Expansion: https://www.nexusmods.com/skyrimspecialedition/mods/37982
Adds new districts and quests to Whiterun.

RubberSpaghetti: https://www.nexusmods.com/skyrimspecialedition/users/36481565
Do you have a toaster? Do you wanna add a little, and I mean little, clutter to towns? Then Spaghetti's town mods are for you. They are super light weight.

Archinatic: https://www.nexusmods.com/skyrimspecialedition/users/10182460?tab=user+files
Maker of the Great Cities/Town mods. These town mods are like JKs but focus more on new buildings/npcs over clutter. Really solid mod collection.

Dawn of Skyrim (Director's Cut) SE: https://www.nexusmods.com/skyrimspecialedition/mods/9074
Sorta like JKs, but more lightweight. This mod overhauls the 5 major cities (Markarth, Windhelm, Whiterun, Solitude, Riften) with both clutter and npcs. Some new buildings aswell. Worth checking out.

JPSteel2: https://www.nexusmods.com/skyrimspecialedition/users/19519279?tab=user+files
Maker of the Cities of the North mod set. CoTN remakes the 4 lesser cities (Falkreath, Dawnstar, Morthal, Winterhold) with custom new buildings and interiors. Really amazing set of mods.
(Note that if you use CoTN it is HIGHLY suggested to use this mod that optimizes the meshes: https://www.nexusmods.com/skyrimspecialedition/mods/85242)

Better Tel Mithryn: https://www.nexusmods.com/skyrimspecialedition/mods/643
Small mod that adds some buildings and clutter to the mushroom town on Solstheim.

Quaint Raven Rock - SE: https://www.nexusmods.com/skyrimspecialedition/mods/20851
Clutters up Raven Rock, perhaps a little to much.

Fortified Whiterun: https://www.nexusmods.com/skyrimspecialedition/mods/40094
Makes the walls surrounding Whiterun complete and sturdy.

Greater Skaal Village: https://www.nexusmods.com/skyrimspecialedition/mods/642
Makes some changes to the Skaal village making it feel more lived in.

Dark's Whiterun market: https://www.nexusmods.com/skyrimspecialedition/mods/22926
Adds a functional little market outside Whiterun by the horse stables.

Solitude Docks Updated: https://www.nexusmods.com/skyrimspecialedition/mods/33777
Adds a new dock/port to the otherside of the archway near Solitude.

Schlitzohr: https://www.nexusmods.com/users/2527296?tab=user+files
Now here's a good one. Schiltzohr makes new lore friendly towns that were in Arena. While nice to look at some of them even come with quests.
(Note that some of the towns quests are unfinished, but a few are functional.)

Ebonvale Settlement: https://www.nexusmods.com/skyrimspecialedition/mods/8472
An older mod, this adds a town near Riverwood. It's age kinda shows.

Rodryk's Dragon Bridge: https://www.nexusmods.com/skyrimspecialedition/mods/42510
This mod gives custom building meshes, and interiors, to the buildings of Dragon Bridge.

SKY CITY - Markarth Rising: https://www.nexusmods.com/skyrimspecialedition/mods/22482
Expands Markarth by building up the mountain side.

JJerem: https://www.nexusmods.com/users/125077838?tab=user+files&BH=0
This man has overhauled the orc strongholds, making them much for fearsome and unique.

Half-Moon Mill - Cities of the North Addon: https://www.nexusmods.com/skyrimspecialedition/mods/64360
Gives the small farm a unique look.

RYFTEN - Make Your Riften - Inner: https://www.nexusmods.com/skyrimspecialedition/mods/63340
A modular mod where you can pick and choose what you want to add to Riften.

Whiterun Has Walls: https://www.nexusmods.com/skyrimspecialedition/mods/101603
Some may think this is overdone, or soulless. It turns Whiterun into a imposing cities suited to be the trade capital of Skyrim.

Whiterun Has Walls Redone: https://www.nexusmods.com/skyrimspecialedition/mods/119229
A remake of Whiterun Has Walls, this version is much more vanilla friendly and is the recommended of the two.

Axis1211: https://www.nexusmods.com/users/42928635?tab=user+files&BH=0
Maker of the Thandome series. These mods add new and overhaul small settlements of each hold.

Thuldor's Ivarstead: https://www.nexusmods.com/skyrimspecialedition/mods/99494
Gives a CoTN style overhaul to Ivarstead.

Enhanced Solitude SSE: https://www.nexusmods.com/skyrimspecialedition/mods/27816
A massive mod that overhauls the exterior and interior of the city of Solitude. Giving it a much needed feeling of being the capital of Skyrim.

Solitude Docks Updated: https://www.nexusmods.com/skyrimspecialedition/mods/33777
Best docks overhaul for Skyrim.

Helsmyrr Village SSE: https://www.nexusmods.com/skyrimspecialedition/mods/76811
Adds a nice little town to Rift hold.

Winterhold Docks: https://www.nexusmods.com/skyrimspecialedition/mods/100889
Adds a small dock area to Winterhold, esl flagged as well!

I hope you found this guide and suggestions helpful. Hope you enjoy the overhauls and new towns.