Ah Gettysburg. My Alma Mater, home town, born and raised
Hans is spot on. Caledonia is nice, Pine Grove Furnace too, both close to the Appalachian Trail but a wee bit of a drive to Gettysburg.
Codorus is nice too, and normally has availability when Caledonia and Pine Grove Furnace often book solid in the summer.
If you want to be closer to Harrisburg, try Gifford Pinchot State Park.
If anyone wants information regarding Gettysburg, message me. I know enough about the battle to bore you in conversation, and a LOT about the region