Don't forget the world class Cherry Blossom Festival, March 26 to April something
http://www.nationalcherryblossomfestival.org/about/
I went once, about six years ago and poorly planned. Others here may have better tips on which Metro stations are good, or which days or times.
Here's a link to the article in today's Washington Post with details on the scheduled events and travel tips for the Cherry Blossom Festival, which will run from March 26 to April 10th. Peak bloom is expected to be around March 29-April 3, but a few warm days can change that drastically! As the time gets closer, the Post will have updates each day.
http://www.washingtonpost.com/local/cherry-blossom-festival-travelers-guide/2011/03/17/ABlLVHx_story.html
My personal experience is that if at all possible, weekdays are much better than weekends, and Metro is the only way to go! I'm quite fortunate that I live close enough that I can watch the weather and when a beautiful sunny day appears, I pack the camera, some water, and hit the Metro for a delightful blossom peeping photo expedition - and the people-watching isn't bad either! LOL!
