From Charleston, you are less than 2 hours from Huntington Beach State Park and Brookgreen Gardens, two of the best birding spots in South Carolina. HBSP is terrific for birders in late March/early April. You can bird (and shoot) along the causeway with a freshwater pond on one side and a tidal salt marsh on the other. Shorebirds and ducks galore, along with a large colony of black crowned night herons. You can walk the mile+ to the jetty at the north end of the park for piping, semipalmated, and Wilson's plovers, along with gannets, loons, and other sea birds. The jetty is best during or after bad weather. Last year I think our species total was about 125, and we weren't working all that hard.
Brookgreen Gardens is a private park with statuary and extensive gardens. Very pretty in early spring. Great birding for migrants in the gardens, also they have a rice paddy with a viewing platform. Plenty of good birds in there (we had great views of sora and king rails right under the platform.)
Finally, there is a local middle school near HBSP which supposedly has red cockaded woodpeckers in a pine tree in the front parking lot. We went by last weekend, but never saw the birds. The holes are pretty obvious, though. You'll need to ask around, maybe on the Carolina birding list, to get exact directions. It's off Sandy Island Road.
There is another SC state park called Hunting Island -- note the confusing similarity of the name. It's closer as the crow flies to Charleston, but it actually takes a little longer to drive there. Also a good birding spot, but if I had to pick one, it would be HBSP.
If you want more information, you can email me privately. Good birding and have fun.
--Ken Bennett