Looked at a lot of military uniforms in the military history section, and First Ladies gowns at the Smithsonian Museum of American History. I would have spent more time in there, but that exhibit is very popular and was very crowded, as you can imagine. My favorites were Martha Washington's 1780s gown, Dolly Madison's 1810s gown, Helen Taft's embroidered Chinese Robe and inaugural ball gown.
Also saw THE Star Spangled Banner - the original flag that inspired the words to the song. It's huge, kept in a dark room (for preservation purposes), and awe-inspiring. The flag-maker, Mary Pickersgill, was paid $405.90 - more than most people made in a year at the time - a good living for a seamstress. Good story on: How the Star-Spangled Banner was Made.