Oh, and I forgot to mention--Baltimore has a great aquarium (National Aquarium) in their Inner Harbor area.
This area also has some great crab restaurants, although the best one is actually 35 miles east of DC (Harris' just off of Hwy 50 on Kent Island). Another intesting stop in Maryland is Havre de Grace. This tiny town is very historic looking--and has the noteworthy distinction of having almost been named the nation's capitol.
(When they were deciding on whether to build a new capitol or just move the capitol from New York, the final two spots chosen were Washington, DC and Havre de Grace, Maryland.)