Standing leg lifts (to front, side, back) wearing ankle weights. Floor work. I would not recommend leg extensions because they can put quite a bit of shear force on the knee joint. If you do try them, keep to a limited range of motion, and be especially careful of not bringing the leg down to tighter than 90 degrees at the bottom.
You can also do quad exercises that are recommended by physical therapists for knee problems (though it may not apply to your case): seated on the floor, leaning back on elbows, keep one leg straight, one leg bent with foot on the floor. Flex the quad of the straight leg and lift it to about 45 degrees (about level with the knee of the bent leg), hold for two slow counts, then lower slowly. The knee joint is stronger and more stable when in a straight position, so this exercise takes advantage of this while reducing stress that can be cause from bending the knee joint. Also make sure to do some hamstring work to keep a good balance between quad and ham strength (hams should be about 70 percent as strong as quads).