One of today's most versatile pieces of equipment is the stability ball. Here are two exercises that can be performed with a standard stability ball:
1) Trunk Curls - Start with the top of the ball beneath the center of your back. Press the lower back into the ball and draw the navel to the spine. As you curl the rib cage toward the pelvis, the torso will begin to curl upward and forward. Hold breifly. Slowly return to starting position.
2) Oblique Curls - Start with the top of the ball beneath the center of the back. Then stagger feet and rotate hips and knees to one side. Anchor the lower hip to the ball and move the rib catge at a diagonal direction toward the legs (ie. right elbow toward left inner thigh). Keep you neck and pelvis stable. Repeat to opposite side.
Besides providng balance training, stabilty balls work the trunk in almost every exercise that is performed on it. So what are you waiting for... get on the ball.