Bring a pair of shorts and a tee shirt, and live in them. That's pretty much it. For evening, perhaps a shirt and jeans to go out in. Most bars insist on closed footwear in the evenings, so take a pair of boots. The golden rule when travelling is to take half as many clothes and twice as much money, but that's easier said than done.
Every journey needs a good book to set the theme, so grab a few choice books to read on the plane.
Essential to every backpacker is a dog-eared copy of Lets Go Australia or something similar. These marvellous guides turn travelling into a global scavenger hunt and are an absolute necessity. I don't have one. I just took some Ken Kesey and Hunter S. Thomson books and followed the interesting looking people around. Yes, Cairns is a tourist trap.