id agree with the numbat suggestion.
you will find Perth Zoo is actually quite a compact little zoo; flat, easy to get around and it wont take you all day. in saying that, if you had a family and wanted to spend a day there then there is more than enough to see, and the path layout makes it really easy to backtrack...
other dont misses IMO...
the nocturnal house and reptile centre. neither are earth shattering in design or particularly modern but they do have a number of rarely seen species, some endemic to WA and the displays themselves are great.
asian elephants
african savannah
silvery gibbons
sumatran orangs
wetland aviary
in fact the whole zoo is good. sumatran tiger, red panda exhibit and a few others arent anything mind-blowing but on the whole Perth Zoo is a great place