Our trip to Sydney was much too short. We only had a few days, and airline delays and resulting missed connections cost us one and a half of those days. Coming from Chicago, we were pleasantly surprised at how mild Sydney's "winter" is. The nice sunny days we encountered were more reminiscent of a cool fall day instead of winter. Your mileage may vary of course. If you have such a short stay, definitely try to stay in a really good location -- Sydney was pretty easy to get around, but it was so nice to be able to walk to the Opera House or the Rocks district from our hotel near Circular Quay.
Food & drink weren't too expensive in Sydney, about the same as any place in the US, and there is no tipping expected. Yay! However, we found many tourist activities to be really steeply priced (e.g. walking tours, bridge climbs), much more expensive than most places we've been. That was true of Australia in general, not just Sydney. But we were able to find more moderately priced (or free!) activities like exploring the Botanical Gardens that were just as enjoyable.
The Ferry dock area at Circular Quay is an enjoyable place to walk around and enjoy the buskers playing didgereedoo. There are plenty of relatively inexpensive fast food places to grab a bite before or after your ferry.
Another recommendation is to take at least a day to visit the Blue Mountains west of Sydney. We enjoyed seeing an different landscape of Australia and it was well worth the trip, even though it meant missing one day in Sydney.