I have issues with my knees too. I like to bike, but I couldn't overdo it or my knees would start hurting again. Swimming is the best cardio for crummy joints. You get a great workout and you can become pretty toned if you train enough.>>6468
You would think this would make it worse but it actually works. I did weight training, so I had to wrap my knees at first, but once I built enough muscle it really helped my pain a lot. After about 6 months I didn't need the wraps anymore either.