I get tender knees occasionally, caused by kneecap tracking. When I do, I sit on the edge of my bed and let my legs dangle. Slowly straighten the knee, then slowly back down. 2-3 sets of ten. It's pretty stress-free, no weights necessary. Like a lot of PT, the repetitions are more important than the resistance. Try it for a few nights, and if it doesn't work you can still see the doc.