My sister and brother in law and their kids were visiting us yesterday, and together with them we went to the San Francisco Zoo. The zoo is rather small, but it has a pretty decent collection of animals. On weekends it gets crowded very fast, so I would recommend getting there as soon as it opens (10am). It is possible to get a free street parking then instead of paying $8 for the "official" zoo parking.
Also, if you think you will like to visit the zoo more than once a year it makes sense to get a yearly membership. It not only allows you to visit the zoo as many times as you want, but also gives you a few free extra coupons for parking, rides on a carousel and even a few free entry guests passes.
We spent around 3h at the zoo and we still did not see all animals that were there. We had to cut our visit short as kids were getting tired and sleepy. In general, I was under impression that the adults had more fun observing animals than the kids :) Below are photos of some of my favorites from the zoo.
Kids on a carousel:
Aditi reading the signs on the zoo bench:
Crowned crane, Uganda's national bird:
Marabou stork and zebra:
Family of Western gorillas:
Mother and a one-year old baby:
Silverback, the dominant male gorilla: