Be sure and go to Fort Moultrie, which is the headquarters of the National Park. Lots to see, including the movie. Fort Moultrie dates far back before Fort Sumter, and played a much bigger part in the defense of this county. I will not go into the reasons why Sumter gets a lot more coverage. If you can find a private boat to Sumter, you can get a lot more time to explore and enjoy the Fort. The expensive boat trip only gives you one hour, and they is lots more to see than that hour will cover.
Go see the Angle tree, and visit some of the plantations. Spend time on the beach. Go see the Fire Museum. The charleston museum, and the aquarium is aolmost as good as the one at Gatlinberg. A trip into downtown Charleston is a real experience.