If they're old enough to tell you, just ask. Give them a list if there are things you don't want them to do, and if you think they'll stick with what they chose for a whole season (being sure to explain the commitment to them), then go with that.
If they can't tell you what they want to do, then pay attention to their interests and go with something they've been into for a while and which you don't see changing anytime soon. For instance, my son gets up and starts dancing whenever he hears music, so I put him in dance class, and he loves it because it's something he was doing already.
at 9:00 PM on Oct. 3, 2010