Antietam is one - the bloodiest day in American history. Then there's a whole bunch in Virginia: The Wilderness, Spotsylvania, Fredericksburg, Manassas (site of two major battles). Further south, Vicksburg, Shiloh, Chickamauga, Stones River are all major sites. You probably haven't heard about any of these (maybe Manassas), but they're among the biggest battles of the war.

The Virginia sites are all huge, historically speaking. Shiloh is a particularly beautiful battlefield. Vicksburg is the most impressive, because it's one of the few with obvious remains of battle - it was basically a trial run for WWI. Most battlefields have a low line of earthworks and some craters, but it's tough to picture what they looked like.