From: Northwestern Georgia, USA
Charleston: Fort Sumter, of course. Also recommended: Fort Moultrie, downtown, and the USS Yorktown (if memory serves, that's the ship anchored there).
Between Charleston and Richmond: Fort Fisher just south of Wilmington, North Carolina.
Richmond vicinity: Petersburg, Seven Days Battlefields, Fredricksburg/Spotsylvania, Wilderness, Chancellorsville.
Because of the huge number of battlefields and other sites in and around Richmond, you probably need more time there than in Charleston, but both cities are great destinations for the history buff.