This squat variation is a great way to keep your core (and all the muscles below the belt) engaged.
Stand with your back to a box or bench that's about three feet away. Bring your left leg behind you and place your foot lightly on top of the box. With your hands behind your head, lower into a squat, keeping your right knee behind or in line with your toes as it bends. Press back up to standing. Work up to 15 reps on your right side, 15 on the left.
Not challenging enough? Add a barbell or dumbbells to increase the resistance.