The absolute best is the following exercise which I got from brother-in-law who actually is a Hospital of Special Surgery trained orthopedic surgeon specializing in joint replacement (and a great guy--my sister did well). It is pretty simple:

After you stretch, put on ankle weights on both ankles, start with 2-3 pounds and you can eventually move up to 5 pounds or higher as needed. Then lie flat on your back with both heels on the floor. Raise your right leg about 6 inches off the ground and draw the alphabet. Draw the letter A, the B, etc. with your foot. See how far you can go. Once you can't go any further drop your right leg and do the same with your left. Do this three times with for each leg. After several days you should be able to go through the whole alphabet and at that point increase the weight and start again.

Why the alphabet? Well your quads (which support the patella) are made up of multiple muscles and the up, down, lateral and twisting movement in drawing the letters strengthens each muscle. This has side benefits for core strength as well.