Just a word of warning. While the Cherry Blossoms are beautiful at this time, do not expect to do a lot of museum tours during this time. We visited during the Cherry Blossom festival a few years ago and the lines for the museums were incredibly long. There were lines of tour buses bringing groups to DC for the Cherry Blossoms and of course those folks were visiting all the museums on the mall.
Having said that, once you see the Cherry Blossoms, there are many things to see and do around DC that are away from the mall area such as the National Zoo, Mount Vernon- George Washington's house, Georgetown, etc.