Q: I have bad knees. What exercises can I do, what should I avoid, and what should I do to strengthen my knees?

A: The standard disclaimer - check with a professional - is relevant here. First rule, do no harm. There are lots of ways knees go bad, some more serious than others. You may be contra-indicated (a fancy word that I picked up and haven't been able to shake, like a recurring cold sore, during my PhD studies - it means "bad idea to do") for some movements and not others. Don't do anything on the say-so of some internet or TV informercial jock.

You may want to experiment with gentle squats, to strengthen all the muscles around the knees.

Also, very important for you will be strenthening the core, so you can protect the knee from buckling if you lose your balance momentarily.

If you like water, swimming sprints will probably be the safest way to get an intense exercise experience.

And believe it or not, I experimented with a running in place playout where I only moved my arms - as fast as I could, pumping them for 30 seconds, then resting for 30 seconds, for a total of 4 reps (4 minutes). I was seriously winded. If that's not a challenge, the Tabata protocol of 20 seconds on / 10 seconds off usually humbles the proud.

Experiment. I'd recommend getting a marble notebook and keeping a knee journal. Keep track of how your knee feels before you exercise, after, and that evening and the next morning.