Sydney – head to the blue mountains, cheap easy access by train.
Gold Coast – has lots of haters, can see why but with teenagers in tow it might be worth a visit. White waterworld and wet n wild water park are a good day out.
Darwin – if you only go for a few days don’t bother with Kakadu, take a day trip to Litchfield national park, 1/2 day boat trip to see crocs and birds of prey with the other 1/2 day spent swimming in water holes. Trip might be seasonal though so worth researching.