Here’s something that looks pretty interesting – earning the geology merit badge (and mammal study) while exploring a cave. They seem to have a nice program designed. Could do all year, not just a summer activity. Lincoln Caverns is somewhat in the area of Gettysburg (center, bottom of PA), so if you are traveling there from afar, you might want to combine the two.

http://www.lincolncaverns.com/groups/scouts/index.html

I’ve been to this one before, just west of Albany, NY:

http://www.howecaverns.com/

They have all kinds of educational programs, they might be able to work with you to design a MB program.

These type of caverns are developed for visitors so they are not the type of activity as mentioned in caving post. My kids loved  Howe Caverns, it is worth the trip.