The area around Circular Quay is home to most of Sydney's attractions. The most famous one is, of course, the Sydney Opera House. It looks like white shells or the sails of a ship. If you come closer, you will see that the different plates it is made of actually are light yellow in colour. They look white due to the reflections of the sun.
Another famous landmark is the Harbour Bridge. You can even climb the bridge like an alpinist. At the foot of the bridge is the oldest quarter of the town, the Rocks. There are all kinds of shops and a tourist information centre inside the nice colonial buildings.
Right in Circular Quay is the excellent Sydney Aquarium (A$14 to A$27), where you can find many species of fish, including a few white sharks. The highlight of the aquarium is the underwater tunnel, where you can walk underneath the fish.