Ft Frederick is just off I-81 in Maryland, Nice old F&I stone fort. (Outside of Williamsport MD)
Gettysburg obviously, Military Museum at Carlisle,
Valley Forge, Filthadelphia, Stop at Ft Chambers Gun Shop at the Scotland Exit off I-81. Small black powder shop. There are some cool minor things, like Girty's Notch where river pirates attacked settlers floating down the Susquehanna. A few miles north of Harrisburg along the west side of the river. depending on when you come through, Blue Mountain Muzzle loaders may have something going on. Shartlesville PA. one of the oldest black powder clubs in the east. Check to see if Daniel Boone's birthplace is open, south of reading PA The cornwall furnace made cannon for the war of independence. The Penn Museum in Harrisburg might be worth a look. And if you have a sweet tooth, Hershey PA is only a few miles south of 81 about 17 miles east of Harrisburg. Yes the street lights are shaped like Hershey Kisses. Travel along Rt 30 through Lancaster county to see all the tourist traps about the Amish, or travel along rt 23 to actually see real Amish.
Most of the mud sales are over before May. A mud sale like Gordonville is a sight. Twenty or so auctioneers selling everything imagineable, including livestock, tractors, furniture nursery plants, quilts, even horses and buggies. Yes Guns too at some.
Any of the many farmer's markets. Roots just south of Mannheim on Tues, the Green Dragon a mile NE of Ephrata on Fri. Renninger's a couple miles south of Kutztown on Saturdays. Zerns near Gilbertsville on Saturdays.
There are a few caves to tour but nothing spectacular. Outlet malls,
BTW we have food outlet stores. They generally sell everything but fresh fruits and vegetables. You never know what they have, Mexican Dill pickles, Cherrios with labels in arabic, They have trouble selling some higher end foods, because the PA Dutch simply don't eat some things. I recently purchased really good goat cheese at one for 99 cents an 8 oz package. Bought Asiago cheese there a few months ago for $2 a pound.
while in the area, try some Lebanon bologna or some bag sausage. If you go to Dixon's muzzle loader shop, you simply must stop at Dietrich's Meats just off I-78 at the krumsville exit. (same exit for Dixons) Try some Birch Beer. It is brewed soda pop that tastes like carbonated wintergreen. Dietrich;s has their own butcher shop and smoke house. Best beef jerky on earth. Folks come from all over the east coast for their PA Dutch foods.