Take the tour at Arlington National Cemetery, see the changing of the guard at the Tomb of the Unknown Soldier. Hit the museums around the National Mall -- but remember plebes usually go to the Holocaust Museum during the summer. The National Zoo and National Cathedral are fine places. My favorites: Arlington, Jefferson and Lincoln Memorials, National Air & Space Museum, Natural History Museum.
Drive down the George Washington Parkway along the Potomac to Mt. Vernon.
Walk around Old Town Alexandria, see Gadsby's Tavern (historic tavern), the Torpedo Factory (old munitions factory turned into art and design home for artists you can watch at work).
Baltimore has a great waterfront: National Aquarium, visit USS CONSTELLATION (last Navy sailing warship), ESPN Zone, water taxi to Ft. McHenry (think National Anthem history) ... maybe the Orioles are playing in Camden Yards, a great baseball venue.
If you are staying in Annapolis and touring from there, I highly recommend driving to the New Carrollton Metro stop about 15 miles from Annapolis on US 50. [US 50 is the main artery DC-Annapolis] Get an all-day unlimited pass, ride the Metro right to Smithsonian, National Cemetery, Zoo and many other stops. Parking in DC during the summer tourist season is a challenge. We arm all our visitors with DC tourist maps (you can probably find one in the Borders or Barnes & Noble) and drop them at the New Carrollton Metro. It's much cheaper than parking in DC, easy to figure out and pleasant to ride.
http://www.wmata.com/
Good suggestion to tour the surrounding area; you will be walking around Annapolis enough in the coming years.