I did whitewater rafting as well as some diving in Cairns and thought it was great. I went diving with a company called Down Under Dive. They do liveaboards on this ship that is permanently out at the GBR and just moves around from dive spot to dive spot. They seemed like a good outfit and offered a full range of instruction as well.
I also did a whitewater raft trip down the Tully river, which is outside of Cairns. It was a lot of fun. I can't remember the name of the company that offered it, but there are several that do the trips. There are travel agents everywhere in Cairns and you can book right before you want to go. Some of the raft companies also offer 3 or 4 day trips where they helicopter you in to remote spots and you raft out all the way to the ocean (avoiding crocs on the lower reaches of the river). It sounded really cool if you have the time and the money.
There are also a lot of different outback safari type trips offered out of Cairns that can involve camping. Good luck with planning your trip and if you have any specific questions on diving or rafting in Cairns, let me know and I will do my best to answer based on my experiences.