Cairns is where you go when you want to do crazy things in a pretty setting. I sky dived, bungee jumped, rafting at Tully rapids and went to the barrier reef. The city of cairns itself is a bit of a tacky souvenir town, full of trinkets, and plenty of bars for night time. I wouldn't go there for culture, but the barrier reef is wonderful even if you are not into the other activities.
Insider Tip:If you have the luxury of time, try to book when the weather is nice, as bad weather can make visiting the reef not only severely vomit inducing, but also affects the variety of species you will encounter. Also if you are prone to sickness, the top advice is to take travel sickness pills frequently days before your trip and during to guarantee an enjoyable trip.