For the last building co-op I asked the kids to bring found objects from home (boxes, toilet paper rolls, feathers, toothpicks, etc...) I cut up 10 pieces of cardboard and painted a road plan on them. On the day of the co-op I gave each child one piece. On it they had to create their piece of the community. They could have buildings, parks, vehicles, people - whatever they liked.
When they were done they were able to put the pieces together to create a community and then play with them as they pleased. My guys put their pieces together and added cars and animals and played with them over the next few days.