I have a bad knee also. Mine is a replacement. I was told by my doc not to use the treadmill because it really puts strain on the knees. I ride a recumbant bike at the lowest level for about 20 min. This keeps the knee moving, gets the blood pumping and puts very little pressure on the joint.

Other that than, if you don't have access to the bike, I will tell you one I learned in Physical therapy that was really easy. You can do it almost anywhere. It was called quad sets. You sit and stretch your knee out straight either on a bed (or support your leg with an ottoman) Then you just tighten the muscles around and above your knee. Try to think of it a pushing your knee into the bed. Hold that for a few seconds and repeat. This is what I did after my surgery to strengthen the quads. It really works and like I said, you can do it almost anywhere.

Good luck