While DC is very interesting, if you want to have fun in your last days before R-Day, I would suggest going to Baltimore's Inner Harbor. You've got the National Aquarium, along with the USS Constellation, USS Torsk, and the USCG Cutter Taney, as well as the Lightship Chesapeake all moored at various points along the harbor. You can tour any or all of them. There are water taxis that you can take to all types of interesting places around the larger Baltimore Harbor, allowing you to get a good waterside view of the city. With the water taxis, you can access the Inner Harbor, Fells Point (the old maritime quarter of the city), Fort McHenry (take the tour!) and other interesting places. If you don't want to get off, you can just cruise around the harbor for a while. There's plenty of shopping, and restaurants in the shopping venues around the harbor, and after a long day, you can walk a few blocks east from the inner harbor and get a great dinner at one of the many restaurants in Little Italy. If you are inclined, you can also catch an Orioles game (if they're in town) at Camden Yards, which is a few blocks west of the inner harbor. The great thing is all of this stuff is within relatively easy walking distance. If you do want to visit the National Acquarium, however, i would suggest getting tickets in advance via the internet if you can.