We also had the good fortune of catching the museum's photography exhibits for Philip Trager and Camilo José Vergara. Trager's architectural photography was excellent, and the prints were very well executed. Vergara's exhibit of storefront churches was less about form and composition, and more about interesting subjects. Both are worth checking out.
Most tourists and locals completely miss the National Building Museum. If you find yourself in DC, you need to make a stop. It's just a few blocks north of the National Gallery of Art and the National Mall, so it's an easy (and safe) walk.