Works your thighs and butt.
Stand with the ball between your mid-back and the wall. Move your feet about two feet away from the wall, with them placed a little more than hip distance apart. Slowly lower your body down into a squat position. Squeeze your thighs and butt and lift yourself back up into the standing position. Push against the ball as you squat and lift and also enjoy the complimentary back massage the ball gives you!
Do 12 reps, rest and repeat.
Also read: Skinny? 10 tips to gain weight