My knee hurts. What excercise can I still do without making it worse?

the inner side of my right knee is hurting. I tried biking since that seemed to be low impact, but it still hurt. Running & biking seem to be out of the picture...it doesn't hurt when i walk, but I also don't want to risk exacerbating it. I really need to excercise since I sit all day at work and feel awful if i can't work out. Is it okay to walk on the treadmill? Yoga? Anything else? I don't have access to a pool, so swimming is out too. Also, are there special stretches I can do? I'm worried maybe I have been working on my quads too much and that might be why i have knee trouble?? any advice is appreciated.