My mom's third grade class recently read a story, Roxaboxen, about a group of kids who create an imaginary community in the desert. They thought it was so cool that, without any prompt from their teachers, they created their own Roxaboxen at the end of the soccer field during recess.
Other third graders have joined in. Their teachers stand back and watch while the 50 or so kids heft logs and branches to construct their community. The actual physical space is not too complex, mostly sticks, pine cones, rocks, and found objects. But little by little the kids are creating a highly complex society, including family units, currency, the media, an airport, and even war (or at least rumors of it). There is a law enforcement agency to keep away mean fifth graders.
The teachers have done nothing but facilitate a blog on the community's happenings. Still, my mom says that they have learned more at recess about how a community operates than any of her previous classes has learned in social studies.