I recently saw these pop up cities linked to from a blog that I read. Here are the versions we created:
I drew a castle on full size posterboard and Lucy colored it in. We made it big enough to fit Playmobil people through the door. When she was finished coloring I used a sharp razor-blade to cut around each piece so that we could stand it up.
Julia wanted a city complete with a store, museum, post office, house, and barn with a silo.
Our pieces didn't stand up well after a while, so the girls put wooden blocks behind each one. These were super easy to make and kept them entertained for quite some time!