The National Zoo is located in Washington, D.C. The zoo has a variety of animals from every continent, similar to many other major zoos in the United States. Its major feature is the great pandas that they recently obtained, and they are a popular attraction with guests.
Admission to the zoo is free, and the two most popular ways to get to the zoo is by car (there is limited parking onsite) or the Metro. The zoo has a number of gift shops and refreshment stands on site.
It is best to allow at least a few hours at the zoo, as there is a lot to see and the terrain can be somewhat hilly in areas. I recommend seeing the National Zoo if your itinerary allows time for it or reserving it as an option if the more popular attractions are booked.