And don't be afraid to take photos of the non-historical things that intrude upon the historic, the juxtaposition of sacred and profane, etc.
Get off the beaten path if you can - there are trails and roads that are open to the public that most tourists just overlook and don't venture out upon at all.
I am certainly jealous - haven't been to Gettysburg in over 15 years and I was but a young teen without a real camera at the time, but thankful my parents were willing to take me around all of the VA, MD, PA battlefields on that week long trip - got to visit at least half-a-dozen fields. I've hit nearly every major site in the eastern theater and most of the western sites except Shiloh, Vicksburg, and anything further west of there. Been a Civil War buff since I was around 6 years old. I may have been one of the few teenagers dragging his parents to battlefields and museums instead of the other way around. I can't wait to get back to all of them again now that I have modern photo equipment and everything is digital.