An emphatic suggestion for US citizens, check your LOCAL state representative and senator(s) websites for tour options. They are FREE of charge and scheduled based on availability.
A few years ago, I had a nice long DC stay and requested every available tour my congressman's office offered.
- The U.S. Capitol
- Department of the Interior Monuments Tour
- FBI
- Kennedy Center for Performing Arts
- Library of Congress
- Supreme Court
- White House Tour
These were not available during my stay:
- Department of State Diplomatic Reception Rooms (IIRC, was not offered 4 years ago)
- National Archives Tour
- Bureau of Engraving and Printing
This year, I'm only going to be in town for the day, requested and secured the all-day Monuments tour again. ^