I picked up something from another user on here called "100 for Core", I think she may have made it up. I do 100 of something every day for core - planks or side plank (I count each second held as 1), crunches, side crunches, reverse crunches, bicycles, the one that I've seen people call 'peeing the dog', boats (got that from yoga and pilates videos), twists and side bends, 'superman fly' with lifts, others I can't remember even the knickname of...

I try to mix it up a bit every day.
ETA - oh yeah, I also tend to do a lot of lower body and ab work on an exercise ball now. Also evil.