There is much to do outside of the National Mall too!
For one there are (at least) two Smithsonian museums not on the Mall itself and open till late (if not later).
1. National Postal Museum
Located just outside Union Station, also great place to buy post memorabilia and/or mail them out!
2. American Art Museum
I enjoyed visiting it - open till late and quite quiet. Also, just a collection of purely American Art - a treat! In addition, they have enough artworks to rotate around in exhibitions.
The Modern Pueblo Painting of Awa Tsireh - I'm a fan. Meticulous, clean and minimalist.
What else not to miss if you are visiting in winter:
1. Christmas markets if you are visiting in December - best when you least expect them
2. Food!
Nothing beats having ramen in the Japan but I like to try things for the hype.
And of course some dessert:
3. Wander the streets at night, find interesting things and get some rest
Onward to Boston!