Toe Taps are a great exercise for the core. Lie on your back, with feet up in tabletop position. Keep your back fixed to the floor, and arms down by your side. Breathe out, and slowly extend a leg until the foot taps on the floor. Then breathe in as you bring the foot back up to join the other in tabletop position. Alternate legs. You can start by keeping the legs bent and tapping near the body. An advanced option is to straighten the legs more, which is more testing on the core.