This is a lovely one day excursion that you can make from Cairns to the small coral cay Green Island. You take the catamaran early in the morning (only 30 kilometres distance and about 1 hour navigation), spend the day there watching the fishes in the submarine observatory in the middle of the largest coral reef in the world, and get back to Cairns late the afternoon, or stay in a hostel in the island (they did not allow me to camp in the island, so I returned to Cairns).
Address:
Coral Sea Islands territory Cairns Queensland Australia
