I agree that gbopp's route is the easiest. Almost as easy would be to exit I81 at Exit 5 in Greencastle, PA and follow Rt 16 thru Waynesboro, Blue Ridge Summit to Emmitsburg, MD and then US15 to Gettysburg. US15 is convenient to Round Top. Artillery Ridge, and Drummer Boy Campgrounds without driving thru downtown Gettysburg. Coming in on US30 would be more convenient to the Gettysburg and KOA Campgrounds. I'd avoid US15 south of Frederick, MD because it is a busy commuter road at rush hours.
No covered bridges on these routes but there are a couple off US15 near Thurmont, MD. I'm curious about where you crossed one between I81 and Gettysburg. Have a good trip.
