Not exactly historic, but Luray caverns isn’t too far off 81 and very cool. And warmer than outside in the winter.
Monticello is a short drive from 81. You could then take US29 north to Manassas National Battlefield, though I’m sure Gettysburg is better.
Williamsburg/jamestown/yorktown is great but far away from your drive.
edit - if you do go to manassas, the Air & Space Smithsonian at Dulles is close by. They have all kinds of planes including the Enola Gay. Then you could swing up to Harper’s Ferry on the way to Gettysburg.
A lot of people don’t know about the museum at Dulles. Links is Steven F. Udvar-Hazy Center | National Air and Space Museum I grew up not far away and several things my Dad worked on are there. It was like having your own personal guide. The museum is free, but you pay to park. And it’s so huge, even if it’s crowded, it doesn’t feel so.