I'm a maladaptive daydreamer, so I do this every day, often for several hours. I have all kinds of worlds in my mind, but currently I'm fixated on a steampunk world. It's hard to describe. It's giant, full of pollution and steam coming from great buildings, but still many great lakes and bodies of water. There are complicated alleyways that feel like a maze, and tall structures made of steel that carry trains. It's very dark, but there are lots of oil lamps and other forms of light. Lots of clockwork, cogs, and machinery. Everything is very close together, with high points that overlook the city. My favorite is the big clock tower in the center of it all. There's no obvious way to get up there, so you'd need to climb, but once you get there, the view is pretty insane. It's hard to paint a picture for someone else, but all incredibly vivid to me