Well, a bum knee shouldn't have to stop you from exercising. There are a lot of good ideas already posted. I have been looking into a program called "Sit and Be Fit" that I saw on TV. One of our channels has it everyday. I found that there are videos and dvd's you can get (check your local library, they may have it). They are aimed at people who cannot get up and work out. I was specifically thinking of my sister who is confined to a scooter. I plan on getting her the tapes and then convincing her to actually use them. There is a website, but I don't have it handy. Hang on, let me see if I can find it...
http://www.sitandbefit.com/sitandbefit/ by Mary Ann Wilson
This website has a listing of Public TV stations and also cable stations that they are on and what states they provide for. You may be able to find it on your TV and not have to rent it.
I also just found another one called Sit and Get Fit by Michael Horn
http://www.sitandgetfit.com/
You could also sit in a chair and do hand weights, like 2 or 3 lb weights to get some resistance training in. Won't do much for cardio, but at least it's still something.
Good luck.
Kathy