There are actually many Smithsonian. Not always totally organized. So do your homework and find out what you are looking for and then seek out that item. Do you want Air & Space or Art? Do you want Natural History and animals or American History and development. Know what you want and give yourself a limit. 2 hours is minimal, but 4 hours will cost you half of a day.
If you own or manage Smithsonian Institution Building, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.