Go down to the Mall and see any of the Smithsonian Museums. All free , all open every day except Christmas 10-5:30. American History (with Star Spangled Banner, Fonzie's jacket, Archie Bunker's Chair, etc is closed until fall for renovations.
Lincoln Memorial, Jefferson Memorial,WWII, Korea and Vietnam Memorials.
Too much to see in one day.
Take the Metro to one of the stops on the Mall.