A variety of crashes. Also, as a kid, for reasons I don't really remember, jumping off our bicycles was the pinnacle of fun for awhile.

Oh, and I very regularly ride (or try to ride) without hands. One or two bicycles were fairly stable, but most were highly unstable.

If you have a hill without anything breakable at the bottom of it, and a bike you don't mind breaking, try pushing the bike down the hill. I don't have any hills here in Florida, but I recall that working pretty well as a kid living in more interesting terrain.