This is a big deal. Erie-Philly is practically an all-day affair to get to one-way. Also how technical do you want your hikes to be? A lot of trails tend to be very rocky, especially the majority of the AT and north-central portions.

One strong recommendation is Lewisburg, PA for a day. Great antique area, R.B. Winter State Park for hiking and some vistas around too, and Rusty Rail Brewing in nearby Mifflinburg.

Another day trip is to Hamburg. Use the AT for the Pinnicle/Pit and eat at Port Clinton Hotel. Yuengling Brewery isn't too far away.