Don't stay to long in Cairns!
Cairns is a nice city but there are not plenty of activities to do, espacially for a small purse. There is snorcheling in the GRB, Skydiving, visit the Tablelands and Daintree forest and that's it. You can discover the whole town in one or max two days. People are all friendly and helpful. There are many backpackers here.
Insider Tip:Visit the travel agencies for discounts or free stuff in the woolshed. Make sure you visit the Woolshed, as it's the best location to go out in Cairns in my opinion.