This was one of the best stops included in our CityPASS. We had a great time at the zoo, and we were there almost all day! The enclosures were very well organized, and we had a great time walking through each area. There were a lot of great areas where we could stop and take a break and sit on a bench and watch the animals as we ate a snack (to avoid zoo prices, we stopped by a grocery store on the way to the zoo and stocked up a backpack full of snacks and water bottles). My favorite time of the day was towards the end when the zoo was basically empty and the weather had cooled down enough to be comfortable. It was really relaxing walking through when there weren't any other people around.
Tip: To avoid the sometimes incredibly long line to see the pandas, go towards the end of the day. We went a half hour before the exhibit closed and there was absolutely no line. We stopped to look at the panda skull and chat with the zoo educator for fun, and then proceeded straight to taking selfies with the pandas. The exhibit was very well set up because the big pile of bamboo was placed right in front of the glass in the mother panda's enclosure. She sat there for the whole time we watched her (probably close to 20 minutes) and ate bamboo.