I'll be in Sydney in two weeks, but only for a day.

Kevin has some great ideas. You might try the Ken Duncan gallery, which is located in the Rocks; it is just a 10 minute walk from Circular Quay. Campbells Storehouse (also at Circular Quay) is a nice spot to walk around and to photograph early in the morning. Take a walk over the Sydney Harbour Bridge - great views of Circular Quay and the Opera House, that are not to be missed.

Regardless, Circular Quay is a great place to start your visit; Darling Harbour, the Sydney Opera House, Botanical Gardens. You can get around the town on the ferry or subway system easily, do don't be afraid to venture about.