I like to use the stability ball. I think Pike & Roll-out are great exercises for your abdominals although there are many more. Pike is where you put your legs on the ball and hands on the floor. Lift your hips up, keeping your hands over your shoulders and your legs straight. Roll-out is where you are kneeling behind the ball. You lay your forearms on the ball & roll the ball away from your leaning forward and putting all your weight on the ball. Then, you press down on the ball as you roll back over your knees. Obviously, there are many more, but these two work well.