Side planks on stability balls, forward planks on stability balls (forearms on ball), kneel on the stability ball, stand on the stability ball, get a mate to through a medicine ball at you whilst you stand/ kneel on a stability ball, do a squat on a stability ball, press ups with knees/shins/feet/toes on stability ball, press-up with both hands on stability ball, press-ups with hands on two small stability balls, press-ups with one hand on a small stability ball/ medicine ball and one on the floor switch hands, oblique (the side of your torso up to your ribcage with your legs in a v shape on the floor) crunches on stability ball, leg extension on stability ball, one leg dead-lifts, one leg squats, and all of your usual exercises that are done sitting can be done on the ball like dumb-bell flys etc. Not really core but ab exercises like stability ball crunches with feet on the floor, crunches with feet on the wall, lie on the floor and grip a ball with your legs pass it to your hands wit your lower back remaining on the floor stretch out horizontally with head and legs just above floor, return to vertical and swap ball from your hands to your feet and repeat. Seated twists basically sit down hold a weight out stretched in front of you with a two hand grip and rotate the torso. That's all I can think of off the top of my head for the moment.
Your best bet is to get to your local gym book a personal training session and ask them to chow you as many core exercises as they know as they need to be done with proper form or they won't do you any good at all as with most things quality is far superior to quantity 100 sit ups don't do anything unless executed correctly.
Iain