A variety of focal lengths is good. Wide angle to cover the blossoms and architecture together, long focal length to shoot/frame around the crowds, and macro/semi macro for close ups of the blossoms (the blossoms aren’t all that spectacular close up, as they are very frilly and undefined).
That said, expect to be able to shoot straight up, primarily, as the aforementioned crowds will drive you crazy. Getting a good shot without someone bumping you or walking into frame is nearly impossible. The popularity of the blossoms has made a visit to the area in which they primarily bloom — the Tidal Basin/Jefferson Memorial — a very irritating and frustrating place to practice photography.
Do take the long focal length, as the last time I was there, there were Bald Eagles in the Tidal Basin area (makes for some interesting shots if you can get a little contest against which to shoot them).
Other than that, there are plenty of photo ops along the National Mall — from street photos of tourists to world class architecture, to nature.