Yoga
Want a really good core workout?
Get into plank position (a pushup position with a straight body), but on your elbows and toes instead of your hands and toes. Keep your body up off the ground and pulse your hips/abs up and down. And if you want a change, you can also rock your abs side to side or rock forwards and back.
See how long you can do that before your abs and back give out....