They have a great rainforest activity where you ride a gondola up into the jungle where there is a little town with a cool zoo where you can hold a real koala bear and take pictures with them and kangaroos. Then had a nice lunch and road an old train back down the mountain. Very fun day.
The next day we went out on a boat to the Great Barrier Reef. That’s the shortest trip to the reef. Great diving and snorkeling.
Port Douglas is a small town but a great spot for a couple days. We flew into cairns from Sydney and had a car take us to port Douglas.
If you go all that far you may want to consider a few days in New Zealand. Amazing country. I’d suggest queenstown. So the shot over canyon jet boats. Very fun. The birthplace of bungee jumping too.