The son of a staff member here at the Dillsburg Area Public Library will be wrapping up his 2000+ mile journey on the Appalachian Trail soon (if not already). While this is a bucket list achievement for many, for others it is only a vague pipe dream. Although the Appalachian Trail never enters York County, it approaches within a few miles of Dillsburg. There are many places to experience the trail nearby.
The easiest place is at Boiling Springs, an AT Community, and the home of the regional office of the AT Conservancy. From Boiling Springs, I have always enjoyed the shortish (6 or so mile) round trip to Center Point Knob, the original halfway point of the AT. You can also access the AT via the White Rocks Trail closer to Dillsburg. There is also a parking lot along PA Route 74 N about 6 miles from Dillsburg (just beyond Ridge Rd.).
A little farther away, but still close by, is the Appalachian Trail Museum, located in Pine Grove Furnace State Park. Pine Grove is approximately the halfway point of the AT, although the literal halfway point varies slightly year-to-year due to trail rerouting. It is tradition for thru-hikers to eat a half gallon of ice cream at the Pine Grove Furnace General Store.