I heard about a merry-go-round used to generate electricity for a school. A quick Gooogle search turned up Empower Playgrounds, a group using kid power to light up off the grid houses and villages.
I also came across an Instructables blog on creating your own swing set for kids to generate electricity.
While the basic plans are free the parts costs are pretty high. I'm thinking though that industrious scavenging can bring that down if you're dedicated.