lay flat on your back, arms to your sides. keeping your legs straight, lift and hold your feet 6" off the ground. hold for 2 or 3 seconds. kinda like crunchs with your legs instead of upper body. You can also do flutter kicks, kicking like you're swimming while holding your feet 6" off the ground. Or holding them 6" up, opening and closing your legs laterally.
edit just thought of another one but you'd need a partner. laying on your back, you have the partner stand right behind your head. you hold onto their ankles, then keeping your legs straight lift them 90* up in the air. your partner pushes your legs back towards the floor, and you hold them before they hit. Those will kill your lower abs.