I skip rope. I like these muay thai-style jump ropes[0] with thick pvc-tube because I find it annoying when a slim rope gets tangled and I have to stop and start jumping again.
Maybe not the best style if you have knee issues though...
Become a triathlete. Swim, bike, or run, usually something will work if you're injured somewhere. Each of course have their own idiosyncracies and overuse injuries.
I either bike around or go on the elliptical or treadmill at my gym.