Start of with something easy like a platform, a dock or a bunker that is a feature in a map, or if you've mapped before or are more confident in your abilities, then try one of these:
Inside a briefcase, would include pouches for storage and assorted folders and papers on the ground.
A kitchen sink. You don't need to include the rest of the room, just make it a high level of detail sink, with pipes and a garbage disposal and stuff you can wander through.
A garage or attic. Both of these have been made before, but it doesn't hurt to have them remade.
Inside a glove compartment.
Inside a volcano.
Mars base.
If you don't like any of those, I can make a list of hundreds.
