if you have not done them before:
Any type of ab Crunches, situps, obliques and variations of them will be good
leg raises are good (lay on your back and raise your legs off the ground trying to keep your core strong and not letting your back arch and either hold them there or raise them and lower them.
i dont know the technical name, but bridging ( move into a position where you are about to do a pushup but rest your bodyweight on your forearms, the only body parts touching the floor should be the toes of your feet and your forearms. the aim is then to keep your core strong and flat for as long as you can).
hope that helps