Christmas Island and Cocos Keeling.
The forgotten islands to the north west are unique to say the least. I can vouch for these two islands as I have sailed right into them both and a great experience was had.
Christmas Island can be accessed from Perth by jet. This is a wonderful place with the best people. Fairly mountainous, it boasts lush tropical jungles as well as coral reefs. There is a golf course that is routinely taken over by tens of thousands of Hermit crabs which provide a unique and mobile hazard. If you are looking for something different, a trip to Christmas Island would definitely be on the unusual. At least this way you can say you have been somewhere that not many travellers get to visit.
The opposite of Christmas is Cocos Keeling. An atoll located far to the west of Australia, it will require a clever traveller to visit this lonely but beautiful spot.