Last year a friend and I mixed some GAP riding with 35 miles on the Ghost Town Trail from Ebensburg PA to Back Lick, north of the GAP. (Journal writeup here.)

Very enjoyable mix (kind of like a blend of the Towpath and the GAP) of history and rural riding, though 1 mile or so in the middle was still in bad shape from floods and may be worse (or better!) now.

Frequent rest stops with lots of railroad, coal mining history and environmental impact, floods, etc.