I vote Calvinball
Although, running around Walmart/Target/Kmart with one of them (aka walking around while bouncing it and thus fighting over it) while looking for other 13" 'playground' balls is fun too. Yep, fighting over them and acting like you're back in elementary school (when you're in high school) in front of random people is always amusing. And those square scooter things from way back when are awesome too (of course)
