Try not to go when it is about 1 PM and too hot...they nap a lot then.Best is early mornings. You get good light then too.
I have a gallery of when I went to our zoo and this was around noon to 1:30 all a bit lazy... have a browse if you like.Greater Vancouuver Zoo
Bring a flash too. Remember how people can be walking in front of your lens...

"The happiness of a man in this life does not consist in the absence but in the mastery of his passions."