Our "zoo loop" goes to the right from the entrance, and we always do the giraffes, zebras, and elephants. They're the biggest most obvious animals, and my son requests them very time. If we still have time and energy (we have a zoo pass, so I don't mind leaving after those), we'll try something else like Living Coast, Tropic World, etc.
I would consider packing a lunch and eat on the lawn by the elephants. Sometimes during lunch time the trainers actually come out and feed them snacks and have them do "tricks" (flap their ears, raise their trunk). It's pretty cool! And even if they don't, you can still see them from the grass while you eat.