Oops--I forgot that I started this thread! Thanks for all the feedback, you guys. I definitely think it's a good idea to aim for a certain number of classes or pushups or something like that, but I really wanted something big and exciting. More like a "one-off," something that would give me a sense of accomplishment after doing it once. The bike and swim ideas are more like what I had in mind, but unfortunately I have no experience with biking or swimming.
I think I might go with Kitathena's idea of speed-walking. I've never done that either, though, and it looks kind of goofy to me!
In search of a lost signature...