Have you tried water aerobics? Your local "Y" usually has them. If you don't have enough money to join, ask them about the free or reduced price memberships for low-income people. They may even have sponsorships that will pay for your membership, or may allow you to just join the water aerobics group.
Also, go to YouTube and look for chair workouts. There are some good ones with bands that might help you.
Tai Chi is very good for those who can't bend their knees. The main rule in Tai Chi is that if your knees are hurting you are doing it wrong. It really helped me a lot, and it can be done sitting as well.