Many rare cultural and historical artifacts are all housed under one roof at the Museum of American History in Washington, DC, one of 17 museums in town that comprise the Smithsonian Institution. More amazingly still, they're all open to the public absolutely free. Whether your interest is art, history, science or pop culture, you'll find at least one museum to intrigue you.
If you go:
Located at the Metro stop for Smithsonian Mall. Ten of the Smithsonian museums in Washington, DC, span an area from 3rd to 14th Streets between Constitution Avenue and Independence Avenue, approximately 1 mile.
Links:

Smithsonian Castle
1000 Jefferson Dr., SW
Washington, DC 200240 USA
info@si.edu
Mailing address:
Smithsonian Information
PO Box 37012
SI Building, Room 153, MRC 010
Washington, DC 20013-7012 USA
Phone Numbers:
Information
+1 (202) 633-1000
+1 (202) 633-1000
TTY (Hearing Impaired)
+1 (202) 633-5285
+1 (202) 633-5285