[masraum]

Swimming should be great, low impact exercise that shouldn't cause you any problems.

I rollerblade. As long as you don't fall, it's not a high impact activity, but can be great exercise. You can go as fast or slow as you like and work your way up. It won't do much for your upper body (although your back may feel it a bit).

I'd recommend checking out a few of the various "classes" at a YMCA or gym or something. Try some out, see what you like and what feels like it's working and not going to kill your knees.