I have a ‘go to’ exercise when I need to strengthen the quadriceps but the knees have long see their better days. It is called ‘Terminal Knee Extension’ and I have a link for you at Youtube which shows how it is done http://www.youtube.com/watch?v=ZscBVtoX62U. Some clients even responded with a comment that this makes their knees feel better instantly.
You should also be able to do all hip exercises. I have found that people with knee problems often have hip problems as well, resulting from weight shift because of favoring one leg.