Do a web search for the Parks themselves. They have visitor centers. I visited them years ago and found Antietam quite worth visiting, and Gettysburg is not-to-be-missed but it is a zoo.

The number of things to be seen in the DC area are so many it is easier to start with the type of thing you want to see first. Since you've mentioned Civil War I'd go over to Richmond Virginia, stopping at Manassas (Bull-Run), and check out the sites around Fredericksburg and Chancellorsville. I was there before the movies came out, so maybe they're a bit built up, but they were just gorgeous and uncrowded with nice visitor's centers and knowledgeable park people, many of whom were guarding the areas in which their great grandfathers fell.

If you are not city bound, Monticello is not too far away and is near the Shenandoah Valley where Stonewall and Confederate Cavalry eluded the Federals. There's also the Blue Ridge Highway and the Smokies National Park. Into Tennessee you've got several famous battle sites, Knoxville, and Fort Donelson, where U S Grant made his bones.