For me, there's no point in exercising if you're not going to work up a sweat. I like kick boxing because you'll be knackered at the end of it, but you feel great. And you'll be strong, too. Try getting a Tae-Bo DVD or something, they're a lot of fun.
Apparently things like yoga, pilates etc. improve your posture, which may decrease the likelihood of backache while sitting at the piano, but if you're like me and aren't satisfied with a workout unless you're exhausted at the end, that's probably not the way to go. I didn't have the patience for it, but maybe that's because I bought a DVD and did it at home. It might be different if you go to a proper class.
I don't think exercise helps my playing at all, but it does improve your general frame of mind, so you may be more inclined to practice.