If you are looking for a great place to take your kids for a full day, the Smithsonian National Zoological Park is a great destination. It provides a wide range of things for the kids to see and do coupled with an amazing experience. I recently visited this location with my children for the third time in the last three years. They are making some changes that should be done around 2013 that will also really enhance your experience.
This is one of the few places in North America that you can see the Giant Panda! You can also catch them on the Panda cams also available through the website.