It's hard to imagine a trip to D.C. that wouldn't include a visit to the National Mall, a swath of national park in the downtown section connecting Capitol Hill to the Lincoln Memorial. Between them is the 555-foot obelisk of the Washington Monument, and the Jefferson Memorial across the still, reflective waters of the Potomac Tidal Basin. Cross the Potomac River from the Lincoln Memorial, and you can visit Arlington National Cemetery, which includes the Tomb of the Unknowns as well as the gravesites of many famous Americans.
Museum Capital of the US
The National Mall is also home to the Smithsonian Castle and museums, including the Museum of Natural History, the Museum of American History, the Air and Space Museum. It's also home to the Museum of the American Indian, The Hirshhorn Museum, the Newseum, the National Archives, the Holocaust Memorial Museum, and within walking distance of the National Building Museum, International Spy Museum, and the Smithsonian American Art Museum.
Although all of these sights are within walking distance of each other, plan on taking several days to see them all. And don't forget to throw in a visit to The White House, as well!