I really like the workout plan in the book Body for Life. I did the program myself a few years back and got strong enough to lift a heavy kayak myself and put it on the roof of my car. He has most of his exercises on his website, and you can print them off to bring to the gym. Here's a link..go to "Trust Your Gut" for a list of core exercises.
Do you know "The Plank" exercise? Research has shown it works better than sit ups!
And any exercise done on the balance ball will strengthen your core as well.
These links should keep you busy at the gym for quite a while!