It needs to be somewhat attractive. That being said I appreciate form following function and function being beautiful so it's not like it has to be some grand structure.
It should be smallish - perhaps a 6' x 4' footprint? 8' x 2' footprint? 6' x 6'? Not much bigger than that.
It cannot be made of glass; I'd prefer no plastic.
It should be relatively mobile; if it could be dismantled and reassembled easily even better!
I live in a wetwetwetwet area in the winter - something that can withstand the wet is a must.
So tell me, internet, what creative ideas can you conjure up? Do you know of a garden shed plan that you've thought would make a great playhouse? Or a play pirate ship that looked exactly like a greenhouse? Or a chicken house perfect for little kids to play in?