The most attractive part of the zoo in Zurich is a big hall filled with a warm and humid rainforest, complete with rivers and ponds. Visitors are not allowed to leave the paths, but there are plenty of animals to see - when you take your time and keep your eyes open.
http://www.zoo.ch/xml_1/internet/en/application/d693/d695/f696.cfm
The foil of the roof filters about two to three f-stops of light, and many of the inhabitants prefer to stay in the shadow. So it's not easy to get a free view on them and good lighting. With my slow telezooms I am using ISO 400 or 800 most of the time, sometimes even ISO 1600 isn't enough. On the other hand, with a little patience and luck you can get pictures without the typical "zooish" look and feel. It's more like a safari, a sweatting hot safari, by the way ... but absolutely worth it!
Tinu