We purchased a 7 day 3 park San Diego pass that included entrance to the San Diego Zoo, San Diego Zoo Safari Park, and Sea World. I would recommend buying this pass (I got it from Costco for a really good deal) because it includes rides on the train, bus, and skyfari and will save you money!
The first park we stopped at is the San Diego Zoo.
This is a big zoo so I recommend you wear some good shoes. There are things for kids to climb and take photos on throughout the park,
Bus/train/skyfari air tours,
and LOTS of animals to see.
You can even see 3 of the few Giant Pandas living in the United States.
There are some animal shows during the day so I recommend arriving to the park early and planning out your day around those. By the end of the day, you might need a cat nap.