There are fast food eateries all over the San Diego Zoo, but the one with the best view is the Treehouse Cafe. The kids' menu is standard zoo fare and I usually get one of their wraps. It's a cafeteria style lunch, but you can take it outdoors to any of the tables at the multi-leveled decks surrounding the cafe. The zoo can get hot in summer, but you won't feel the heat with the breezes blowing through this area.
Hint: I've seen plenty of families combine some of the zoo food, with snacks they brought from home.