On our Sydney visit recently my wife and I had a couple of hours to kill before we needed to go to the airport so we jumped on a Sydney Ferry. They criss cross the harbour going everywhere and being public transit are really cheap. If you go to one of the smaller destinations like we did you don’t need to get off and when you tag on and tag off at the same Ferry Wharf you pay the minimum fare even though you went to the other end of the harbour! We went to Olympic Park from Barangaroo we had a great time cruising the harbour on this sunny afternoon like rich people :). The same tour on a tour boat would have been around $50 each. We paid just over $3 each :).
Paul