We did a post day in Brisbane, with the
ABD transfer dropping us at an airport hotel where we were picked up about 30 minutes later for a full day of humpback whale watching. It was spectacular and great fun, with many many sightings, breaches, and the boat also has audio from the water when they are singing. June/July is migration season with calves, and they are intense. That night we visited the area and then took the hotel shuttle over to the airport. It was a nice way to end a fabulous trip.
If you are more adventurous, we took an OYO day and went tandem skydiving on the Gold Coast beach from the Gold Coast airport. They provide pickup and return to the hotel. There are some pictures, etc. in our trip report on this already posted in trip reports.
Australia is a great destination. Enjoy!