Bridge.

Tones your hips, buttocks, thighs, back, and abdomen
Doing the Bridge against the ball is a completely different experience than doing it on the floor. With the ball, you'll really feel your hips and buttocks working to keep your torso stable.
Sit on the ball with your knees bent at 90 degrees and your feet flat on the floor. Place your palms on the ball for support and slowly walk your feet forward, allowing your back to slide forward along the ball.
Release your hands toward the floor and continue to walk your feet out until the ball rests under your shoulders, as shown, supporting your shoulders, neck, and head. Reach your hips toward the ceiling. Hold for 1 minute, breathing normally. Inhale as you walk your feet back in and rise to the starting position.