For a killer ab workout, try doing crunches in the pool. To do water crunches, float in the water on your back perpendicular to the side of the pool. Put your legs up on the deck of the pool up to your knees. Lie flat in the water, then use your abdominal muscles to pull your upper body up out of the water as far as you can. Use your muscles again to lower your body back into the water without splashing. Do 10 to 15 repetitions and rest. The water will help you get a better range of motion than doing regular crunches on the ground.