Last time we came north along the coast, we stopped at Savannah (the historic section is beautiful) and Charleston (to see the city, the Hunley and Fort Sumpter) the took the two boat rides (one is inexpensive the other is free) over to Hatteras. That leads you to Norfolk and Virginia Beach. Then you can also stop at the Mariner's Museum in Newport News where they are working on Monitor parts. You are still on your way to DC and the Smithsonian but parking is a bear, then National Zoo a little better. If you choose to go on to Baltimore to the National Aquarium, that is a good stop. Philadelphia is full of good things to see and we haven't even gotten to New Jersey yet and you are probably out of time.
Matt