A variation on the "against the wall" squats that you can try is a stability ball squat. Put the ball between you and the wall, and squat down to parallel. Instead of doing reps, hold at the bottom position for 20 seconds (or however long you can). Repeat this a couple of times. Add on more time, until you can hold for 1 or 2 minutes at bottom position. Watch that you keep your back in neutral position and don't arch it against the ball or round it.
The wall (or wall/ball) squats are nice because you can move your feet forward a bit (especially with the ball squats) so that your knee is a bit behind the ankle, which helps take pressure off the knee. Of course, if this move hurts, don't do it (but it's used for knee rehad by PTs).
Another rehab exercise that works the quads is supine leg raises. Start with no weights. Lie on your back, with knees bent and feet on the floor. Straighten one leg on the floor, then slowly raise it until the knee of the moving leg is at the same height as the bent leg. Hold for a beat, then slowly lower. Sometimes it helps if you point your toes outward a bit, to get more of the quad that is on the inside of the knee (vastus lateralus? or is that medialus? Or is that even spelled right?). Work up to about 3 sets of 15 reps, adding ankle weights as you improve.