Painting a City!
I've been playing some 15mm WWII games lately and one of the scenarios we wanted to try was one on a 2 foot square board, the terrain being entirely made up from buildings, so as to get a Stalingrad-type game.
However, since we were totally lacking enough buildings for anything this ambitions, I went ahead and made some. Nothing fancy - just blue foam with holes in stuck to mounting board with a hot glue gun.