Stability balls cost about $20 and are available online or wherever exercise equipment is sold.
Bridge Roll (Not recommended if you have back or neck problems)
For more of a challenge, extend one leg toward ceiling and roll ball using only one leg at a time.
B Rotate legs and ball to right, lowering right leg. Hold for a second, then rotate to left, raising right leg and lowering left. Hold and repeat. For more of a challenge, hold legs at a 45-degree angle to floor and rotate.
Lie on back with legs straight and heels on ball. Raise right leg off ball and grasp calf or thigh, pulling leg toward head until you feel a gentle stretch. Hold for 20 seconds and release. Repeat with left leg.