We flew into Cairns but stayed up in Port Douglas(acutally just outside of town at the Thala Beach Lodge). Obviously, the big draw is heading out to the Great Barrier Reef and diving or spending the day at one of the many islands off the coast. There is also the Skyrail Rainforest Cableway just outside of Cairns, which is pretty cool.
If you make it into Port Douglas, catch the toad races at the Iron Bar. Good times betting on the toads. Drink plenty of 4X beer..
cairns and port douglas are only places on your list i can attest to. spent two weeks in port douglas. best diving in the world. so many things about australia make it such a great destination spot. you cant go wrong with a place that has both great beaches and great beer. and they like americans, and are extremely friendly in the bars. fun people.