Feet shoulder width apart. Raise one hand straight over the head and with the other tilt to the side trying to touch as low down on the leg as possible. Repeat on opposite side. 10 reps or more.
another is; feet shoulder width apart, hands behind the head. twist the torso as far to the right as possible without moving the feet and then reverse it. 10 reps or more.
Along with the crunches, these give a more complete abdominal workout. Repeat both several times during a session.