You can also do squats (and one-legged squats) with a balance ball. The ball is sandwiched between your lower back and the wall. Your feet are shoulder width apart in front of you (you should be leaning back into the ball, not just standing against it - like using a Smith Machine for squats).
Squat from this position - the ball will roll down with you - keep your back straight.
To use only one leg, place one ankle on the opposite knee, then squat.
Using one leg will definitely give you a weight boost!