Harbour Boat Tour
Hop-On Hop-Off Bus/City Sightseeing and Sydney Harbour Boat Tour
For first timers, I highly recommend you take both the Hop-On Hop-Off bus touring around Sydney and then the Sydney Harbour Boat tours, since the city is quite large and made up of a few islands and harbours.
They are both fun and give you a good picture of this exciting city, especially when you travel with young children.