I use to be there regularly and there are lots of things to do and places to visit. The main attractions of are Harbour Bridge, Opera House, Bondi Beach, Torango zoo etc.
As the place is surrounded by water, cruising will be a best option, you can opt for cruises with different providers like, Captain cook, Australian cruise group, Vagabond cruises etc.
But I am not able to explain more without knowing your interest. Kindly share your interests and I can guide you more.
Thanks and regards,