Learn to Dive
If you get certified (or licenced) to dive on the Great Barrier Reef you can then dive all over the world!
With both PADI & SSI courses available in Cairns you have lots of options. Most courses will begin with 2 days of lessons in the "pool and classroom", followed by either day trips to the reef or some time on a liveaboard dive boat. There's even an option to get certified in just 3 days - all of them at sea.