My clients often feel that more is always better. I make it a point to educate them that this is not the case. Low impact activities like cross-country skiing, yoga, Tai Chi, rock climbing, and pilates are all good ways to keep knees strong and healthy, burn calories, and increase cardio muscle strength without overloading the joints. Biking is a particularly great low impact activity for knees. It moves your knee through a fairly large range of motion, with resistance it will build strength and if you do enough, it will help you to lose weight.
In addition to these suggestions, a good physical therapist is a must if your pain is not changing. The longer you wait, the longer to recover. A good physical therapist will be able to educate you on how to restoring range of motion, strengthen, and give you a customized program to improve YOUR individual mechanics.