We did a move in bellydancing this eve that might help, stand tall with your pelvis tucked and pull in and push out (in v controlled way) your belly button, you very quickly feel it in your stomach muscles!
lie flat on your back lift your legs roughly 6 inches of ground and do little kicks up and down without letting your feet touch the ground or go to high! one of the toughest ab exercises you can do!
also the plank is good kind of in push up position but with your elbows and forarms resting on ground and your back straight.. do for 30 secs at a time and that will strengthen your core!