Harpers Ferry will take you maybe 3 hours, Monticello, where you have no choice but to tour with a guide, will only take a couple of hours.

We did your exact trip in reverse. There was plenty of time to do everything. The only thing you might run into is lots of traffic on the 3rd.
We spent the night in Harpers ferry at a Comfort Inn (or was it Days inn?)...totally boring, but close enough to wander around wihtout the car.

It was definitely a good idea to book ahead for Gettysburg. I went in late October once and the hotels were even difficult on the weekend. The Antietam-Harpers Ferry-Skyline Drive day will be a very busy one, but you might be ok. Your husband would probably love Antietam. It is even more authentic than Gettysburg and without the crowds. The drive to Antietam from Gettysburg is about an hour, maybe a little more. Whereas it can easily take a day to do any justice to Gettysburg, you can see Antietam fairly well in 3-4 hours (2 minimum). The drive to Harpers Ferry is between 30 minutes and an hour. You can see Harper's Ferry in 3-5 hours and not miss too much. That time of year the days are so long that you should still have a decent amount of daylight for the mountains. Get an early start. You may want to reserve a hotel in the mountains somewhere to be safe. We came through in late October from Gettysburg, planning on staying in the mountains one night on the way home to North Carolina. The hotels were booked up almost all the way to Charlottesville, so we saw Monticello instead of Skyline Drive on our last day. Of course that was autumn in the mountains, but the July 4 holiday period could be similar, especically considering the 5th is on a Friday (a lot of long weekend trips for people). Take a look at my Web site at www.oneeyed.homestead.com for information and photos from Gettysburg and Antietam. There is some info but no photos for Harper's Ferry.