Well, crunches would do that if you put heavy plates on your belly. But have you ever seen a marathoner with a bulging belly? Light and many, those are key. Few and heavy bulks up. Unless you are talking Bud light, then many of those and you get a bulging belly.
Arnold Schwarzenegger's book, don't ask me the title, states that you should exercise the abdominals, the forearms and the lower legs everyday. This is because they are used to do anything at all on a daily basis, so they are the hardest to train to what you desire.
So I am not talking doing 30 crunches with 100 pounds on your belly. More like doing 300 crunches with varying techniques morning noon and night.
Other good ones to try, stairs, walk them. Or go dig holes, like in the movie. I bet those kids got in really good shape.