Kicking with fins adds resistance targeting your butt, thighs, and hips. You have to kick harder, so you'll tone up faster than with no fins at all. Wear rubber swim fins ($28) like these when swimming laps, or just hold on to the side of the pool and practice kicking. You'll be able to feel the difference right away. Not only will you tone your legs and booty, but you'll also become a stronger swimmer.