A common situation is that the party is in a big city (maybe one you created yourself, maybe one from a published source) walking through the streets, just roaming around.
And then the question is asked: "What do I see?"
I always have trouble deciding what they see.
Most time I say a tavern, blacksmith or a clothes shop, but that feels very boring to me.
How can I get better at improvising shops and other facilities on the spot during city exploration, to increase the variety of description, and maybe open new possibilities for the party?
Best Answer
You don't have to improvise--just use the DMG's random tables.
The DMG (p. 112-116) has a "Settlements" and "Urban Encounters" section with tons of rolling tables to randomly generate buildings and encounters in civilized settings.
For example, the "Random Buildings" section has details for taverns, warehouses, and residences: you can roll a d20 for a residence and get "hidden slaver's den" or "front for a secret cult".
While you will probably still have to improvise a bit in order to fit these into your town, these tables will help inspire you in coming up with interesting encounters--it's always easier to come up with things if you have a prompt.
Generally, you should know the city or region very well, and it should be unique for some reason.
I have a bit of a bias for running urban campaigns, and so I've run a number of them in the past. The key aspect of running an interesting city or town is knowing what makes it unique, and then demonstrating that to the players.
If you're making your own city, you need to come up with its traits: is it a dwarven city? Is it a trader's city? How old is it? How rich is it? If your players are particularly inquisitive, it might be worth pre-generating a few demonstrative encounters. For example, in order to indicate that a particular city was high-magic, I created an encounter in which someone was being attacked by spontaneously animated objects. Stumbling onto this situation gave the impression that magic was common and unpredictable in the city.
If you're running a pre-written city, this work is probably done for you. Consider Sigil, a city described in the DMG (p. 68):
With this, you know that the city is very high-magic and full of commerce. When players ask you "What do I see?" you can respond with a brief description of traders carrying strange, obviously magical objects, or a market that doesn't seem to be selling physical objects (because it's information being traded).
Improvising adventures is hard
Some people might say that they only ever wing it, but I find this incredibly difficult (and you probably do too, if you're asking this question!). It's a lot easier to shift actual adventure creation to prep time. You can prepare a handful of adventures, and then give each one lots of hooks to make it seem more organic.
For example, if there is an evil slave trade going on that the players want to stop, there would be evidence of it in many places--people in chains, rumors flying around, a shopkeeper assuming that the party wants to see his "secret wares," and so on. Then, the players can happen upon one or more of these things, and end up in an adventure if they choose to pursue it.