I hope I'm not hijacking the thread, but my wife and I are also heading down there for the first time (next weekend, arriving Friday night and leaving Sunday afternoon).
We want to check out the Wormsloe Plantation for its beautiful live oak trees and Bonaventure Cemetery for sure, but otherwise we're mostly just going to take it easy and see what we can see. Maybe visit a beach on Hilton Head.
I think we're also going to do one of the ghost tours (Oglethorpe has one where you can apparently get out and explore the "Candler Hospital Morgue/Tunnel"), and maybe a riverboat.
Any advice about recommended tours (ghost or otherwise) would be appreciated; there are so many different tour companies that it gets a bit confusing.