This dynamic core exercise from Tamilee Webb, star of the Classic Buns of Steel DVD series, will really rock your abs (and probably make you smile too).
Sit with your hands just outside of your hips, fingertips forward. Bend your left knee, foot flat on the floor, and extend your right leg out straight, slightly lifted off the floor. Brace your abs in tight and rock backwards, bending both knees, rolling onto your shoulder blades, pressing your hands into the floor behind your shoulders. Quickly rock back up to your seated position (with your right leg extended), and use your abs and arms (not momentum) to lift your hips slightly off the floor. Hold for 1 count, bracing your abs in tight, and then rock back and repeat. Do 10 reps and then switch legs. Try up to 3 sets.