Our DD2 lived in Charleston for about 8 years and graduated from the medical university. It is one of my favorite cities to visit. All the best things to do have been mentioned. I just loved walking the old cobblestone streets and soaking up the atmosphere. You can really travel back in time. DH and I both picked out our dream house on the Battery. I have pictures of them in our bedroom. Magnolia Plantation is the best. The gardens are extraordinary. I would love to go back there some day.
ETA: If you like antiques, don't miss the shops on King Street. I saw things there we never see out here. Too expensive for me, but fun to browse. And if you want low cost good food, ask the locals where they go. They know the little out of the way places with good food.