Rivnuts from Homestead, PA on 12/16/2019 10:40:11 AM:
That's a new and intriguing question. Which direction you plan to make the trip and the weather during the trip would likely impact your choice. That said, here are some thoughts:
1. Cumberland, MD is the largest town directly on the trail and offers multiple lodging and food options. The town itself is not particularly overwhelming but you might find a fall excursion trip up the hill on the Western Maryland Railroad to Frostburg, MD to be nice.
2. Ohiopyle, PA is a colorful stopover in the fall and a side trip to Frank Lloyd Wright's Fallingwater home is a rather unique architectural entity.
3. Harpers Ferry, WV offers an opportunity to visit one of the well known sites from the Civil War. It's a small, quaint little historic town.
4. Great Falls, MD has an impressive set of waterfalls in the Potomac River and a working canal lock to see how the canal was traversed in its day. However there is no lodging at the falls themselves.
5. While not logistically consistent with a layover/rest day, the two terminus cities of Pittsburgh and Washington, DC offer all sorts of diverse sightseeing options which you should take advantage of it you have not visited one or both cities.
These are just some of my thoughts. Depending on what your interests are, there may be several others. I'm sure other contributors to this forum will have other suggestions as well.