Cairns is more of a party town, I suggest going to PD, the sheraton there is cheap with C&P. To get there I rented a car but we were 4 guys, and driving, either you can do it and get used to it right away or not. There's also a bus for 40 AUD pp that you can take.
In the Sheraton there's a activity desk with a girl that knows the area really really well. I called her up before and setup the first day and went to see her when I got there to setup the rest. (You can charge all the activity's to your room and even use points) I went on the "posideon scuba/snorkeing boat (there are a few diff options) ours was really amazing!
I would also make sure to bring a poncho and bathing suits, it rains a lot there for 10-15 min at a time, so we just wore bathing suits all day.