We spent the morning with my parents at the National Building Museum. It's basically an impressive building that was saved from destruction, and is now used to host exhibits.
The biggest claim to fame is that the great hall is used for the innaugural balls.
What I found most impressive was how the building was made out of cheap materials (millions of bricks) but made to look like it used fancy ones (marble columns, for example). Like so many things in DC, it's more facade than anything else.