Juggling: It's awesome because there is a vibrant community pushing the limits of what's possible. There are so many different things to juggling (balls, clubs, rings - But also devilsticks, pois, diablos, yoyos, ...) and you can always learn something new. It's also pretty easy to start and you can learn new tricks fairly regulary, so (at least in my experience) you don't hit a wall that often. One very cool thing is that you can get started with 3 tennis balls or other objects and, if you think it's worth it, migrate easily to "real" juggling balls and other things that cost more.