Weighted Sit Up: Do a normal sit up while holding a five-pound weight in each hand, reaching your arms straight past your knees. If you're stronger, keep the weights closer to your body, resting on your chest or held up near your forehead.
Spiderman Contractions: Start in a push-up position with your arms shoulder-width apart. Keeping your core tight and maintaining your form, bring your right leg up to your right hand, creating a lunge postion. Quickly and carefully move your right leg back to the plank position, keeping your body in a straight line. Repeat with the left foot.
Standing Knee Raises: Standing with your belly scooped in and your elbows behind your head, bring your right knee up, twisting your body to bring your opposite elbow to your knee. Think of it as doing bicycle sit-ups while standing!