(Here are the 'rules'.)
We have a PE bin in the coatroom upstairs, loaded with conventional equipment like basketballs, frisbees, air pumps, and soccer balls, as well as an unconventional collection of broomsticks, snowshoes, stomp rockets and the like.
When we play Calvinball, each student takes a few items from the bin. Then I put them in groups on the playground or in the park and tell them to invent a game. Then I stop those games and tell them to change all the rules. After that iteration has gone on for awhile, we rejigger the groups and they make a new game. Then they combine into larger groups and invent something new.
Here is the game that Ada, Lindsay, Matthew and Marcellin invented. It's called 'Calvinball'.