Planning on a trip to Washington DC and will be hitting some of the Smithsonian Museums (Flight, Space, and Natural History for sure), and I really want to take only one lens so I don't have to mess with carrying a lot of gear. I usually carry around my 28-300, but I also have a 24-85. Nikon D750, so I can crop if necessary, but do I need the extra reach?
I imagine the 24-85 will cover most of everything inside any of the museums on the mall, which include several Smithsonian buildings and the two National Gallery of Art buildings, and at the very least the great majority of everything else you might want to shoot. There's also the US Botanic Garden just east of the southeast corner of the mall, across a street or two, with a lovely and huge greenhouse ... if it is open.
About four blocks north (and slightly up hill) of the mall - between 7th & 9th Streets NW - you could visit the Portrait Gallery and the American Art Museum which have been joined into essentially one building by a covered atrium. They usually have one or more photo exhibits at any given time, and other items of interest as well, not to mention several lovely restaurants in the area.
If it is the sort of thing you find interesting, I'd suggest you try some architectural studies of some of the buildings from the outside as well as the inside. I have found the American Indian Museum, the East Wing of the National Gallery of Art, and the Hirschhorn Museum pretty interesting, and, while I haven't shot the African American History and Culture Museum yet, it looks pretty interesting architecturally as well.
If you're going in warmer weather, there are some lovely gardens behind the Smithsonian Castle (which houses the Visitor Center), between the Arts and Industries building and the Hirschhorn, and some other spots on the mall. And there are two sculpture gardens on the mall, one in front of the Hirschhorn Museum, and the National Gallery of Art Sculpture Garden directly across the mall from the Hirschhorn.
I've spent many an hour shooting and enjoying the contents of the exhibits, the buildings, and the gardens, and every time I go back, I am sucked into it all over again. I don't think you can miss having a spectacular time. Just be sure to take several memory cards!