I have tried swimming and it is a great way to excercise. My problem is getting bored! Thats one of the reasons I like Racketball. It's challenging and keeps me interested. I find that it is easier on my knees then running. My cadence while I run is about 90 so that means I'm hammering each knee about 3 times every 2 seconds. Racketball isn't nearly as repititious.
It does of course take two people to play though. Something else I like is trail riding. This is awesome exercise. I try to ride at least every other Sunday and each Sunday when I can. Try doing 40 to 60 miles on a dirt bike, It's great exercise!