Some of the city's best museums are not on the Mall! The East End is home to many more of the Smithsonian museums, including the National Archives, where you'll find the U.S. Constitution, and the International Spy Museum.
The three branches of the government are located just around, but not on, the Mall. The White House is just north of the Washington Monument in the West End; the must-see Capitol Building and the Supreme Court are just east on Capitol Hill.
The most famous war memorial in the city, Arlington Cemetery is not actually in the city—it is just across the Potomac from the Lincoln Memorial, across the Arlington Memorial Bridge in Arlington. At the northern tip of the cemetery is the marvelous Iwo Jima Memorial; at the southern tip, the Pentagon.