PS - The exercise ball I got was from Marshall's. It was only $12.99. Marshall's actually has a good selection of small weights and things you can keep around the house to work out with.
The Jack Hammer
Put the ball under your shins and get into the plank position.
Bring the ball in using your lower abs. Do 3 sets of 20.
Lower Back Lift
Put the ball under your pelvis and hang over the ball with your hands on your head.
Lift up and hold for 3 seconds. Repeat. Do 3 sets of 20.
This move is the most effective ab workout. It works the 3 main muscles in your abs all at once. If you want to challenge yourself add a 2lb weight to each leg.
Basically you pretend like you are riding a bike on the floor. Keep the legs low. Do 3 sets of 25.