From my many trips to that area.
You have the aquarium at Fort Fisher, I love it myself, and kids really love it. You also have the museum at Fort Fisher. Take your time to not only go through the building, but the grounds as well. One of my favorite things about that place is the "wall" that was built into the Cape Fear, so that the ships had to take a certain path, thus making them vulnerable to cannon fire. Built entirely of slave labor!
You also have the beaches of Fort Fisher and Carolina Beach you can drive on, right down to the water.
Further north, you have the USS North Carolina. If you have never been there, it's a must see. Gives you a really good idea how life was on one of the greatest war ships ever built.
Oh I about forgot, down at Fort Fisher, you have the Fort Fisher Ferry system. It's a short 30 min ride over to South Port. Buy you a loaf of bread or some crackers to feed the gulls that are out begging while you wait on the ferry and follow along while you are on the water. Kids love feeding the gulls. Many will come take food right out of your hand.
Hope this helps, have fun!