I do a lot of core work on a stability ball (crunches, pikes, rollouts), do decline crunches holding a heavy medicine ball, do a lot of my general weight work on a bosu ball which works your core while you are doing other exercises.
The sorest I have ever been (think major surgery sore) is from doing 50 rollouts on a
powerwheel. Not only were my abs excruciatingly sore, but my lats and traps were screaming for days. I don't ever want to be that sore again!
I also managed to cripple myself a few years ago doing decline crunches while tossing and catching a 10 pound medicine ball vertically. Toss up as you come up, catch as you go down.
Mel