For a quick trip like that, just stay in the area. There's some great dives available in the Brisbane area itself -
http://www.godive.com.au/Day_Dives.htm - and you can drive / bus it a hour's north to the sunshine coast for a great wreck dive on HMAS Brisbane at Moolollaba (various operators).
The best barrier reef dives really need a liveaboard, and hence 3-4 days, unless you're willing to pay $$$$ for options involving low-level flights or helicopters. Save that for your next trip, and just enjoy some good dives near where you'll be this time!