How about a "tent" that fits over a standard card table? They have patterns for them.
You could make a postal theme with a mailbox, mail slot, a envelope and letter, mail jeep on one side of the tent, US flag, etc.
Or a fireman theme.....firetruck on one side, the pants and a fireman coat on one side...hoses and axe and fire hydrant, etc.
One of the moms demonstated hers at a seminar that I attended....she just went with whatever the child liked. She put a door and windows in the various sides and got really creative. She actually took a picture of a mailbox and copied it exactly....also sewed a postage stamp (larger of course)...