Sydney is such an easy town to navigate. If you dock at the further pier (where most of the smaller ships dock) then just take a taxi into town and explore. There are ferries that take you to Manly Beach, Toronga Zoo, …. Walk the circular quay, explore old town, walk the botanical garden, stop for an amazing flat white coffee. I would only take an excursion to the Blue Mountains for a day but we used a taxi who drove us all around the area for a reasonable price.
There are no post cruise excursions but I have seen a note on the website that they do set them up for passengers who are continuing on.
Enjoy this beautiful city!