I canoe and kayak. I actually prefer the [single person] kayak over canoeing, much more maneuverable and fun IMO. I actually do more canoeing, however; because I don't get the opportunity to kayak as often, but I love it.
Probably the most fun activity would be white water rafting. The best course I've ever done is the Royal Gorge in Colorado, it is amazing. It can get real rough, especially when the water gets high. They actually raised the age to raft the Royal Gorge a few years back to 16 because they had a few deaths of children. It can get pretty dangerous depending on the water level, but it's great fun.