A few of my favorites from the PA area not already listed:
York Heritage Rail Trail: runs from York PA to the Mason-Dixon line at the MD border (about 20 miles or so). This trail links up on the MD side with the Northern Central Rail trail which will take you as far south as Cockeysville MD (another 20 miles).
The Western MD Rail Trail runs through Hancock MD and about 10-12 miles east and west from there. Paved, flat and nice views as you head west. Parallels the C&O Towpath.
Oil Creek Trail in Oil Creek State Park is a beautiful ride (You can round-trip that one in 20 miles)
Sandy Creek Trail (from Fisherman's Cove to Van, PA) is a favorite of mine in that same area. Roundtrip is about 25 miles; bit of a grade at the eastern end.
Something a bit different: Presque Isle Bike Trail in Presque Isle State Park in Erie is a fun ride. You go around the peninsula (about 13-14 miles) with views of the Erie bayfront on one side and views of Lake Erie on the other. Not actually a rail trail, but a ride you need to do at least once.
Enjoy!
Last edited by SpokeEasy; 03-13-14 at 09:28 AM.