The zoo is trying to keep up with the times. They are currently installing an elephant walk so the elephants can roam "free" in a "natural" setting rather than being cooped up in a circa 1950s space. It will be worth returning in 2013 when that is finished.
As we did not arrive until after 4, we were pleased to find out the buildings stay open until 6. After leaving the pandas we went to visit the orangutans. The last time we visited the zoo (a few years ago) we were mesmerized by the work being done in the Think Tank: http://nationalzoo.si.edu/Animals/ThinkTank/ Here scientists and researchers are testing the orangutans' memorization skills. It is fascinating research. Even without the "show," it was still fun watching them play.
The grounds stay open until 8. We still had lots of time to see some other animals. None are as cute (in Ashley's eyes) as the pandas.