We went to Charleston 2 years ago for Spring Break with DS13 and had a great time

. Charleston really had something for each of us along with good weather, even though it was only mid-March (early Easter that year).
In the end, we stayed at Wild Dunes Resort which is a very nice resort community on the beach north of Charleston. This place is your typical East Coast beach resort with pool, tennis, golf, bikes, great beach - all the activities you could want. Even though the water wasn't warm at all, there were kids in the ocean all the time and DS spent a fair amount of time riding waves brrrrr

. The adult contingent were content wading along the shore and the weather was fine for that. From the resort we took an environmental tour on a small boat thru some of the estuaries and inland waterways in the area, saw lots of porpoises and other wildlife / waterfowl and spent some time wandering around an uninhabited island. Lots of activities like that are available in the area.
From Wild Dunes we were ~ 20 minutes into downtown Charleston, very easy to get there. We took a tour out to Ft. Sumter, wandered around downtown and did some walking tours and checked out the architecture, and took a horse drawn carriage ride. Also went in one evening for a walking Ghost Tour which was GREAT fun

. Another really interesting spot is Patriot's Point Naval and Maritime Museum. Highly recommend this! It's on the Mount Pleasant side of Charleston Harbor. The aircraft carrier Yorktown is anchored there as well as a submarine, destroyer, Coast Guard cutter, and several other military ships - all open for touring. Even if you're not particularly interested in this type thing (and I'm not) it's definitely worth spending some time there. You can't imagine what it's like to wander around an aircraft carrier - talk about a HUGE ship - and going down into a submarine is hard to describe.
We all thoroughly enjoyed Charleston and will probably go back again some time. If you're there a little later, I'd add in visits to plantations and gardens.... azaleas typically begin to bloom in late March and the area is spectacular then!