Smithsonian Institution Museums
This world famous museums includes not only 19 museums but also research centers around the city. Ten of these museums span an area from 3rd to 14th streets between Constitution Avenue and Independnece Avenue, almost a mile. So be warned and make sure you are wearing comfortable shoes when visiting the museums.
The Smithsonian includes National Museum of Natural History, National Museum of the American Indian, National Zoological Park, Arts & Industries Building, and Freer Gallery of Art. It would be advisable to look up the museums to find your area of interest if you are short of time. All museums are open every day of the year except for Christmas day and admission is free.