The Baltimore Aquarium is pretty awesome, and on your way. It’s $25, but it has lots of sharks.
Annapolis is a little out of your way, but it’s incredibly cute and quaint, with more of a laid-back, old-timey feel. If the weather is nice, walking around there is a great way to spend a few hours.
If you decide not to take I-95, you could go see Luray Caverns. It’s about 50 or 60 miles due west from DC on route 66, and then you head south a bit on 81. It looks like 81 goes directly southwest from there all the way to Asheville, but it seems to be all in the mountains. I’ve never driven that way but Luray Caverns is pretty cool.
There are also a lot of local wineries in the Rt 66/ Loudon County area, which would be on your way if you took 81 after DC. I’ve been to Naked Mountain, Delaplane, Oak Barrel Winery and a few others in that area. Virginia wineries have an odd collection of signature grapes, like viognier, cab franc, and petit verdot.
Or, if you continue down I-95, stop in Richmond! If you take the Broad Street exit, the first thing you will see is some shitty, run-down industrial spaces and a baseball diamond with a giant Native American caricature man literally bursting out of the side. If you don’t see anything else, this will tell you all you need to know about Richmond. You can drive down Monument Avenue, where all the monuments are to fallen confederate generals. The one exception is a statue of Arthur Ashe, where he’s surrounded by kids and it looks like he’s about to hit them with his tennis racket. The guidebook says it’s a must see.
Richmond has a couple lunch places that I’d highly recommend. Edo Squid is a fresh Italian seafood place where they douse everything in a shitload of olive oil. Ipanema has vegetarian food that’s actually pretty substantial and satisfying, and every server has an ironic mustache tattoo. Buzz and Ned’s is a great ribs/ soul food place near the previously mentioned industrial wasteland/ baseball diamond. There are also a lot of cool galleries on Broad Street, and cute/ quirky shopping in Carytown. Or you can go to Belle Isle (access from Belvedere Street) and walk across the river on the bridge under the bridge.
Richmond also has its own winery, James River Cellars. There used to be a very aloof cat that hung out in the tasting room, but he has mysteriously disappeared.