If you are a sporting buff, then I'd recommend a tour of the MCG / Docklands.
Both tours take about an hour and cost <$20.
The redeveloped MCG is a sight to behold, new members area is incredible.
I don't think they've finished the new sport musem precinct, however. If they have, add about another 90mins to your visit (based on what the old Olympic museum was like.

mt