While you're hunting for bullet shells on the battlefields (they're still fairly numerous), please stay clear of the rubes in Manassass. I live about 15 miles away, and boy are they toothless.

DC has very nice, free museums.

On your drive down, stop somewhere on the Eastern Shore (Maryland) and eat some crabs.

The best coffee you'll have here will probably be in Florida at a Cuban place. My favorite is in St Augustine (on the of the most beautiful but touristy places I've been to here.) If you get around to it, try the Colombia restaurant. I recommend the 1905 Salad.

As you move further south from Boston, notice how much more friendly people get with each mile down. By the time you get to Georgia you will have people worrying about how your day is going and if there's anything they can do for you while you're around.

Hope you have fun.