A mess generally.
I build stuff from Starships to bridges on my half of the dining room table.
I have a couple of areas where I build my terrain, but I don't have any photos handy. One area is my home office, I've got two corner desks from a modular system, that are pushed against each other and that's where my computers are at. Then I have a straight modular section which is where I do my modeling as far as assembly and painting is concerned.
When I'm casting parts, currently only using dental plaster, I set up a table in either the living room or clear off the dining room table and put down a plastic tarp to contain spills. I've also got a card table set up in the living room temporaily, till my wife gets tired of looking at it, which I'm using to build an inn using foamcore, wood, plaster bricks and cardstock for the flooring.
For painting large areas, I usually do that outside, setting up some saw horses with a piece of plywood on top, instant table! I'll need to snap some new photos and post them if I get the time and can remember to do it.