I glimsed this gorge from the train windows over 10 years ago, and have wanted to return ever since.  There is a little trail, with a platform stuck onto sheer cliff walls, that looks unbelievable unsafe and exciting.  

http://www.andalucia.com/antequera/chorro/home.htm

I hope you check it out and tell us about it!

Cairns has the most dive boat options; here are 3 I've liked. All have resort-dive options, for first-timers

1) Reef Magic - typical medium-size boat, amenities, friendly crew, can get in two dives if not too crowded. Can get a video of yourself diving, photos boarding, but like most everything in Australia it's low-key.

http://www.reefmagic.com.au/ (hmm, seems more commercial now)

2) I took a small sailboat with 6 pax, 3 crew, and stayed overnight on the reef. Unique experience. Can't remember the name, but you should be able to find it or something similar by asking some knowledgeable booker (booking places all over Cairns). Book this early if you can, as soon as you know your dates, it can fill up. I got a spot a few days ahead of time but it was the low season.

3) Big boat, day trip to Fitzroy Island (I'll try to stay overnight there next time). You can make a quick stop there and then go on to a dive spot, but I spent several hours, took a snorkle trip near Little Fitzroy Island. Islands might not be novel to Seattlites, but to me it was very cool.

www.fitzroyisland.com.au/

In general, you can find great diving/boating experiences if you look a little deeper than the very common short little trips.