Ditto all the suggestions above.
The guided tour in Gettysburg is excellent and combining it with Amish country is an excellent idea.
Baltimore has the Inner Harbor-tall ships and the National Aquarium, but if it were me, I'd spend my time elsewhere. Annapolis is beautiful and historic and nearby.
DC- the spy museum is fun and near Ford's theater. I highly recommend going to Mount Vernon. There's so much to see in DC you'll have to choose what not to see!
If you want to see a different part of the rural east, consider Monticello and/or James Madison's Montpelier, which has only been restored in the past 15 years, and is a little closer. It is a bit of a drive, but a beautiful one.
It all depends on what interests you. Good luck deciding what to do.
If I were to pick just one additional destination, probably Mount Vernon, for it's history and scenery. But also add Amish country to your Gettysburg tour. Take highway 30 east from Gettysburg and you're there.
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