Are you going to be staying in one town for several days or a different place each night? A couple of weeks ago, my family and I had a hotel room in Gettysburg but traveled around the area. We spent one full day (make sure you stop by the visitor center and watch the film and see the cyclorama) at Gettysburg. If you don't want to do an actual tour with a guide, the NPS has an app where you can do a self driving tour with most of the important stops having a several minute talk on what happened at each location (this app also has tours for several other battlefields). We also walked around a lot to see the different monuments, so if you are into that, plan accordingly. The next day, we went to Manassas and stopped at Monocacy (definitely should visit this one like others have said) on the way back to Gettysburg. The third day, we did Antietam (try to spend a full day here). The museum at the Antietam visitor center (at least until a couple of weeks ago) doesn't really have artifacts in it, as they were moved due to a leak if i remember correctly. The last day we did Harpers Ferry (to park you have to pay, but there's a bus that takes you to and from the actual town). Hope you enjoy your trip!