Oakland Zoo isn't a world class zoo by any means, but it is a fun outing for the family. There is enough of a variety of animlas that you won't get bored, and their new children's petting zoo is sure to delight the younger members of your family. All of the standard animals are here (elephant, lion, tiger, monkeys, snakes) as well as a good selection of more exotic ones.
The worst part about the zoo is the rides. You are almost forced to exit into the rides section and of course these are not covered by your admission ticket (even the zoo train). So you will end up shelling out extra for your kids to experience the rides.
If you plan on going more then 2 or 3 times in a year it makes sense to get an annual pass, especially since the pass includes free parking.