If it's a daytime trip, the Toranga Zoo was really neat --
If you have a few hours, I don't know if it's done in the evenings, but the Bridge Climb is superb. Basically you are taken on a tour of the Sydney Harbor Bridge (It's worth it to do it, but you will be breathalyzed before you go on -- so don't drink before going)
There's a casino, probably more than one, I don't remember the name or the address, but it was near the AMP Centerpoint tower.
I loved the harbour cruises that left from Circular Quay, they went to different places for just a few hours.
if you like Opera, the Sydney Opera house is worth going to - it's pretty building. The opera company itself is average, but the building is impressive.
Near Darling Harbour, (I think - I can picture in my head where it is, but I am not 100 percent sure) there's a very cool Chinese Garden, that was fun, was just a nice Zen way to spend the day.
Paddys Market - -not far from the train station, great place for picking up your touristy knick knack crap for dirt cheap.
The Rocks -- nice walking area, has some trendoid restaurants and bars - -but its also got a ghost tour - that was ---boooo -- fun
http://ghosttours.com.au/
Pick up the Syndey Morning Herald in the morning, it used to list lots of things to do in the evening. In the time I spent there, I was never just stuck in my hotel room -