Core strength underlies everything we do. Without it, our cardio work and our other strength work comes to very little. We need it to stand, to balance, to control our movements, and to protect us.
We all know about crunches (I hope). Here are a few other exercises to try when you are bored with them:
Pushups and Pullups
Planks, Straight and Side
Medicine ball overhead slams and oblique slams
Kettle Bell Twists
Straight Leg Lowers
Anything with the TRX