Exercise for bad knees
I also have bad knees, and my orthopedist recommended cycling. It strengthens the quads, which helps stabilize the knee joint, and is aerobic without the damage of running.
I am finding that the rowing machine also is good. And, of course, swimming (but I find it easier to get to the gym than to a pool). Strength training is good, if you work with a trainer to create a routine that doesn't stress the knees, and use appropriate weights.