Searching FT will show many past threads on this topic. Great diversity in what is available, but if you look at Heron and Lizard (cheaper, more pristine and eco-centric) and Hayman and Hamilton Islands then you will get the range of choices. Cost varies from $100 to $1000+ per day so quite a range. Also big tradeoffs in ease and cost of travel (Hamilton has airport and easy access, while some other islands may cost $1000 for a helicopter transfer or take a day for boat transfer). All depends on your time and money tradeoff, and whether you are looking for nature vs. man-made attractions.