lying leg raises would be good too.
as in lie down on your back on the end of something so that your legs are dangling off the end (like your bed or something), hold onto something with your hands to keep you from moving, then raise your legs.
legs are heavy so your ab mucsles will have to work hard to lift them, it'll probably be hard if ur not strong in the ab department (like me) but i'd say that'd be quite good to do considering it employs the most basic idea behind making a muscle larger (ie. use the muscle to lift something heavy repeatedly until you get tired).