Try Fitzroy Island....
Not sure of the prices and times this year but last years info...
$25ish for the ferry leaving around 1900 from CNS. 40 min ferry ride to fitzroy island backpackers resort. On a beach, palmtrees everywhere, extra bars etc and a target rich environment. There would have been several hundred people there.
Ferry returns to CNS at 0200 and 0300. There is another one back at 0900 as well if you are unconscious or otherwise occupied on the beach in the wee hours.
Enjoy!
CS