Tandoori's suggestion are spot on!! The first two exercises that came to my mind. They are VERY effective and also brings up the heart rate.
Some more ideas: Holding weights in hand, stabilize your stance in wide but slight (horse) stance and box (straight punches, upper cuts) alternating fast and slow movements.
Bicycle: Flat on back, lift knees to 90 deg angle, lift head to crunch position and bicycle. Alternating fast and slow, as well. I've also noticed that working past the burn as long as u can stand it, makes me more sore. This one hits ALL areas of stomach!!
A variation on the leg lifts: One back, lift legs and torso off the ground, straightening as much as you can without losing balance. Hold that and clap hands over head (100 Xs), then clap hands over abdomen (100 X's or as many as you can stand). Then repeat except with legs bent in 90 deg angle, this targets upper abs, though.
Lastly, sorry so many, but they keep coming to mind
Regular old sit up, except punch punch (opponent on either side of u) across your body as you come up.