Personally, i don´t use restrooms or food stands.
Prairie Dog and normal restaurants work good for me...also, keep in mind that the aesthetic value of your zoo (ej. the roads, observation areas and decorative fences) will increase the guest happiness if they look good. For example, sometimes, i use rock or atlantean arches in the entrance for increasing happiness; also, don´t forget to use the Longhorn Statue and the Turtle fountain (surround them with flowerbeds).
An extra tip:
T-Rexes, Dolphins, Sharks, Orcas, Velociraptors, Tigers and Elephants will increase your guest happiness. Also, many popular animals will make a good looking zoo.