Try this plank with knee drop to tone and strengthen your core.  From all fours, move into a plank with hands splayed and fingers pressing into the mat.  Press back through your heels to distribute weight more evenly between arms and legs.  Without dropping your hips, lower your knees to the floor and then straighten your knees again.  Exhaling as you lower the knees and inhale as you straighten.  Repeat this until you feel the deep work happening in your core.