The best thing for knees is biking , a crosstrainer machine/eliptical strider , and swimming.
Also getting out of the house to exercise helps more than staying in , unless you are anx about outside. If you enjoy walking try doing it on grass/mud instead of hard sidewalks.
But ANY exercise works WONDERS
exercise always helps, best if done in a relaxed way, with no excess muscle tension
walking is very good, if you live in a bad area, go to a safe area to walk, some walk early morning when few are around, some at night
when I was mostly housebound I walked late at night when few were around