Cape Cod to Washington DC - help & advice please!

My boyfriend and I are planning a trip to visit my friend on Cape Cod in September/October for 2 weeks, and are hoping to take 5-6 nights out of the trip to do a road trip down to Washington DC. The plan would probably be to spend 2 nights there, and so probably allow 2-3 days to get there and the same to get back, so that we have time to a bit of sightseeing on the way.
Obviously I know that NY, Philadelphia and Baltimore are all more or less en route, but is there anywhere else you'd recommend to visit? I also have mixed feelings about NY, I've been there already and I absolutely love it, but my boyfriend hasn't and I kind of feel that your first trip there should be a proper one and not just a quick day trip. But on the other hand, it seems a shame not to visit when we're going right past it, given that normally we live on a whole different continent and can't easily just drop in!
Any suggestions would be much appreciated!

Ceud mìle fàilte! Welcome aboard the RoadTrip America Forums!

Well, there's certainly a lot to do in that area, but I think that first you'll have to decide what to do about New York. While it would be a shame to pass it up, I absolutely agree that it is a tough city to see in one day. You could spend that much time just getting in, finding parking, seeing one or two 'sights', retrieving your car, and getting out. One alternative would be, perhaps, to take a tour of New York Harbor or around Manhattan Island from New Jersey. Stops worth making other than those you've already listed would include Mystic Seaport in Mystic, CT and poking around both Amish country around Lancaster, PA and the Eastern Shore of Maryland. Annapolis, MD is a great walking city, and I believe that you can still sign up for tours of the Naval Academy. If you have specific interests, just let us know.

AZBuck