Traditional squats are an excellent exercise for working your abs, but this single-leg variation will have them working even harder to stabilize your trunk as you lower and lift.
How to do it: Stand on your left leg, right knee bent and foot lifted off the ground, holding dumbbells by your sides. Bend your left knee and squat down, hinging forward at the hips and reaching dumbbells towards your left foot as right leg moves slightly back to help counterbalance the body. Fully extend your left leg to return to start. That's one rep. Do up to 3 sets of 10 repetitions on both legs.