I saw people playing Bubble Ball, or Knockerball, in Seattle last August. Players run around in big PVC plastic inflatable balls that you can see through. You can ram one another and when you fall it doesn't hurt... but it is a little hard to get up. They're designed so you could actually do a somersault if you want, so the ball could become a tool instead of an impediment. It's fun to watch and apparently fun to play.

[SOURCE: Tri-City Herald]