U.S. Capitol and Capitol Visitors Center, Capitol Hill. Enter Visitors Center from the east side of Capitol. Even if you only see the Visitors Center it is worth the visit. The Capitol Tour includes ear phones with docent led tour beginning in the Rotunda. Try to obtain ticket from your Congressman prior to departure or go online; otherwise stand in line upon arrival. Gallery passes of the House and Senate will need to be obtained from the office of your Senator or Representative.
Supreme Court Building - First St & Maryland Ave NE. East of the Capitol and next to Library of Congress. Free lectures when court is not in session.
Library of Congress - 101 First St. SE. Next to the Supreme Court Building. Enter on the west side, ground level. A must see. Beautiful architecture and changing exhibits. Guided tours.
Botanical Gardens - At 1st and Maryland SW, immediately Southwest of the Capitol Beautiful, tropical and nice respite in bad weather.
National Museum of the American Indian - 4th St & Independence Ave. Western hemisphere native communities. Great art. Good introductory film. Restaurant.
Smithsonian Air & Space Museum - 6th St & Independence Ave SW. More visitors per year than the Louvre. Wright Brothers 1903 Flyer, Lindbergh's Spirit of St. Louis, Apollo 11 lunar command module. IMAX movies, Planetarium and flight simulator. Restaurant.