The Bicycle Crunch is a great core exercise. It is tough because there is no rest, but you can add rests in and build up in your own time.
Start lying on your back, hands on head (keep elbows wide). Lift legs up and hold in table top position, and crunch up. Twist from side to side aiming your left elbow towards your right knee, straightening the opposite leg, then your right elbow towards your left knee. Keep the shoulders lifted for 30 in a row.