Try decline twisting sit-ups. Decline twisting sit-ups are more intense than normal sit-ups, as you are working against gravity as you lower the body and rise up again. Just perform a set of normal sit-ups, but twist to alternate sides as you rise from the floor and reverse the position as you lower back to the start position.
Lie on your back with your knees bent and your feet firmly set on the ground.
Raise your torso, focusing on twisting as far to one side as you possibly can.
Lower back to the floor slowly, as this helps work the muscles across the full range of motion.