Scott introduced me to a nifty software toy: BallDroppings, available for Mac, Windows, and mobile phones, plus a Java version. All free.
The idea is that a series of small balls drops from the top of the screen, and you draw lines on the screen for the balls to bounce off of. Each bounce makes a sound (with pitch affected by various factors). The balls are influenced by gravity and air friction, which you can adjust. The result is a pretty, entertaining, noisy piece of art/music/fun.
There are more instructions on the BallDroppings page. Note that when you launch the app, your screen will go black; to draw lines, just click and drag. You can also drag the endpoints of lines you've already drawn, and you can delete existing lines. You can even save a configuration and restore it later, though there's only one save file and getting to it on the Mac (to make a copy of, or to share with others) is a little tricky.
Anyway, it's cool, it's fun, it's pretty, and if you're really good/careful it may even be musical.
One thing I don't understand about it is the bounce algorithm--it's not the usual "angle of incidence equals angle of reflection" thing, so it can be a little hard to predict which way a ball will bounce. But that doesn't really detract from the fun.