J.G.
A visit to the Smithsonian Natural History Museum is always a hit with my kids. We have been multiple times. It is huge so don't forget to visit the bug zoo. We stayed in this section alone for at least a half hour. If it isn't too cold on that day you can ride the carousel which isn't too far away. And if you are up for the metro ride there that is an adventure in itself. (my kids also love the air and space museum and the native american museum)
Enjoy your visit!