The Smithsonian is the world's largest museum complex with more than a dozen free museums in Washington, DC. Its own spread around the National Mall, and has a name of its own such as the Air and Space Museum, the Museum of American history, and the natural history Museum.
I was so widespread and the number of collections from each of the Smithsonian museums, you would not be able to explore and enjoy the beautiful creations in just a day. But since this place is free, you can go back as many times as you like without fear of dry bags.