I want to add one more piece of advice I wish I'd thought of before we went. Either take out a subscription to Smithsonian magazine or go to the library and read through about the last year of issues. Throughout the magazine they will have in-depth articles with background on some of the displays at the various museums throughout the city (and country for that matter). There are also shorter blurbs with brief highlights. One can find out about some of the current exhibitions you may not know about. I know once we returned and I got some back issues to read, I was disappointed by some of the things we missed.