It's worth going out to the zoo simply to have an excuse to take the ferry. And the ferry ride is excellent. As you set out the Opera House is on your right in all it's splendor. And to your left is the magnificent Harbor Bridge. If you look closely you can even see groups climbing along the top of the bridge. It may help you decide if you want to give it a try later.
As you get out a ways the skyline fills the sky and looks even more magnificent. Anywhere you look you'll get great views across the harbor.
Now the zoo itself is no slouch. It's located on a hilltop and offers excellent views back across the harbor to the city. The surrounding vegetation adds a lot to the idyllic setting as well. And then there's the animals, from all around the world. This is an excellent zoo. It doesn't have the space that say a San Diego Zoo, Toronto Metro, or Brookefield (Chicago), so don't expect it to be in that ballpark. But it's a very good zoo with a wide variety of animals both Australian and from other continents.
Nonetheless it was the Australian exhibits that I found most impressive. There's an excellent platypus exhibit, and we were lucky enough to see these animals active. You can actually walk around in the same enclosure as the kangaroos. And for an extra $2 you can get right next to a koala and get your picture taking with it.
There are in fact several presentations and photo oportunities. You should get a schedule of these when you enter. There are also several special tours of the zoo you can take. Some you need to sign up for in advance. For that kind of info check out the website right here.
The Treetops restaurant near the elephant exhibit proved to be a delightful place to have lunch. Very good quality food, perhaps better than you'd expect in a place like this. It's licensed as well.
A good way to explore the zoo is to take the skyrail up from where the ferry drops you. It will take you up to the top of the hill. From there you can wind your way down to the lower gate and you're only a short walk from the ferry.
All in all this will be a visit to the zoo that you won't forget, and I'd say almost a must do. It definitely is if you like zoos.