Good walking shoes. Water. Camera. Check the NPS for activities and ranger talks. I managed to score a small slab of a witness tree they were cutting down when I visited (also made to take the Loyalty Oath despite my habit of carrying medallions of President Lincoln (a penny).
You might want to read up on the industrial revolution and gunmaking. They've a lot of belt powered machines there including a blanchard lathe (for making gunstocks). The reason why the pattern is twice the thickness of the finished product is they don't want the stylist (object that contacts the pattern and controls the cutting on the workpiece) to "push" the pattern and result in a deformed end product (I was taught to use a router powered stock duplicator).
I was just there yesterday. The Civil War museum was closed for renovation. Didn't say when it might reopen. There are no trashcans in the park except the restroom.
And since you're so close to Frederick, MD, I suggest you check out the National Museum of Civil War Medicine. It is awesome.
Also about Harpers Ferry, the bookstore is amazing.
I would say plan a multiple day trip. Spent about a day there in 2021 during a group tour. One day isn't enough. We did Arsenal Square, the railroad bridge and Bolivar Heights as a group. Me and another attendee went to Harper Cemetery during the lunch break. So much more to do and see.