Niagara Falls?
Gettysburg for sure.
If the two of you are the type that can handle it, more power to you! When my wife and I were considering our honeymoon we began with planning a road trip (prior to gps). Then one day as I was contemplating maps, and getting lost, and traffic jams, and carguments, and the possibility of a car accident in unknown territory, and a different motel - hotel - B&B each day, etc. etc. (These days you may also want to think on the probability of meeting bed bugs) As for me, I squashed that very quickly. We went to the Caribbean and loved every minute of it. It made it possible for the focus to be us together instead of where we were going next and what time we had to be there.
Niagara Falls is a full day’s drive in each direction, well N from DC on the NY/Canadian border, so may not fit in with this trip. Gettysburg (PA), on the other hand, is little more than an hour away from DC and in the general direction as Harper’s Ferry WV that I’d include, time permitting.
VA has Williamsburg, Charlottesville (Monticello and UVA); Bedford, VA has the D-Day Museum. In Alexandria VA, close to DC and Arlington Natl Cemetery, the “Old Town” section is charming; it has the Tomb of the Unknown Soldier of the American Revolution in the graveyard behind the Old Presbyterian Meeting House; and it is only a few mile N of Mt. Vernon, which is near Woodlawn Plantation and Gunston Hall.
If you belong to AAA, go to one of their offices, and their staff should be able to help set out what’s along the way. My daughter went to Tulane and every Spring/Fall, we’d do a road trip between NOLA and DC, it was great! If you want to go all the way to the East Coast, there’s Charleton (Ft Sumpter and Ft Sullivan), Savannah, Hilton Head, then cut across to Atlanta. Whole lotta wonderful USA to be seen. The whole region should be gorgeous in October, and neither too hot nor too chilly.
Lots of luck, Chuck, on your wedding trip, we wish you and your bride-to-be a long, happy life together.