This move was recently named by the American Council on Exercise as one of the very best thigh toning exercises, but it also engages your abs (you'll feel it the next day!).
How to do it: Loop a resistance band around your ankles and lie on your right side with your right arm extended on the floor, left hand in front of your body for support. Brace your abs in tight, bring your bottom leg slightly in front of your top leg (let the inside of your top foot rest on the ground), and lift your leg up to the ceiling, keeping your hips stacked. Maintain tension on the band at all times and pulse your bottom leg up and down quickly 20 times. Repeat on the other side.