Gettysburg.
I've been to Fredericksburg (overdeveloped), Spotsylvania Court House, Chancellorsville, The Wilderness, Richmond (only parts of it), Petersburg (only parts of it), Five Forks, Appomattox Court House, Sailor's (or Sayler's) Creek, Yorktown (American Revolution oriented), Dam No. 1 (on Yorktown Peninsula), Murfreesboro, Richmond (KY), Antietam, Shiloh, Kennesaw Mountain, Franklin (Carter House & adjoining areas), Lookout Mountain, Pickett's Mill, Cobb (Kulp) Farm, Fort Donelsohn, Fort Sumter & Moultrie, Knoxville (where Col. Sanders, not the KFC dude was killed & the Fort named in his honor), Secessionville (where my car was attacked by a Reb dog - I had gone too far, put it in reversed and backed away at which point the Reb dog gave chase. I stopped the car to turn it around and the Reb dog rammed me. I couldn't believe he was that stupid. He walked away.) and Nashville. Even been to Bowling Green, KY where the campus is site of Confederate entrenchments. Gettysburg is one major site (along with Vicksburg, New Orleans (are those forts still there?), Mobile Bay (anything there?), Port Hudson, and a whole host of others) I've yet to visit. Is Ball's Bluff still around or is it developed? I know Big Bethel on the Peninsula is overdeveloped.
BTW, if you want to see a well preserved coastal fort and get an idea of what Sumter looked like, go to Fort Point in San Francisco. It's open only on weekends now.