If you're a local or planning to visit the zoo three times or more, buy the Zoo Membership. Family of four can buy the package for about $128 (one visit at regular price would be $104). See http://www.sandiegozoo.org/membership/
Good place to burn off some energy if you skip the bus tour. Without the bus tour, I don't think you could see the whole park, but you do get to spend more time watching the animals you like.
The Elephants, Gorillas, Polar Bears, and Panda are the big draws, but nearly all animal have good displays. I like the Polar bears and hippo's display as they are setup so that you can see below the water. There is a sea lion show about mid-day that is hokey but fun.
Bring a lunch as the food inside the part is expensive. You can get your hand stamped for re-entry later that same day.
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