Last few days of the beautiful gardens.
The Smithsonian Institution Building
The Institution Building or "Castle" is the original home of the museum. There are a number of great features that make it the best place to start a day on the mall: a scale model of the mall, clean bathrooms, and a small cafe. There's a nice intro video that runs on a loop (starring Ben Stiller), and the crypt of Joseph Smithson (starring Joseph Smithson). The best are the mini-exhibits from all of the Smithsonian museums. It's perfect if you've ever wanted to check out the Smithsonian but not deal with all that walking around.
As always, I was most interested in the pop culture offerings like these gloves and helmet from the Miracle on Ice.