Another vote for hanging leg raises. If you're gonna do the medicine ball slams, make sure you use the soft leather balls not the hard rubber ones...ouch. If all you've got is the rubber ones you can stand about a few feet apart from a partner and toss them into their stomach. But you don't want to be slamming those down on your partner like you can with the softer ones.
Should look up Freddie Roaches Abdominal Exercises for Boxers on youtube (billiardjay). A fairly good workout, comes in 2 Parts and 1 other part for lower back if your keen.