Last month we took the kids to the Smithsonian National Museum of Natural History in Washington D.C. The national museum is free for everyone and a great place for the kids to see dinosaurs or great replicas of wildlife. My oldest especially enjoyed the dinosaur and ocean exhibit. If the Butterfly exhibit is still running you should go and check it out. You walk through a small enclosure filled with butterflies flying around and snacking on pieces of fruit. My oldest thought this was so neat and when you walk out an attendant is there to make sure you don't leave with any butterflies.
A great place to take the kids and the older they are the more they enjoy all the exhibits!