I booked my tickets at the zoo entrance, and they were confirmed instantly. I just scanned the code to enter, and they were exactly the same as the official tickets, but much cheaper. There are all sorts of animals and shows; you could easily spend a whole day experiencing them all. If you're short on time, just see the main attractions or things you like, such as the classic view of the giraffes by the sea alongside the Opera House and Harbour Bridge. The round-trip boat ride offers various views of the Opera House and the CBD skyline.