Going on a 3d/2n liveaboard trip out at Great Barrier Reef is the way to go. You'd have around 10 dives including 2 night dives. It's also not just limited to scuba divers, snorkelers can also join the trip. A tip when booking your trip, the liveaboard price is cheaper if you book it in Cairns. Just have a walk around town. The price is around $50-100 cheaper than booking online or with any travel agent.
Cairns Situated In the North eastern region of Australia and Is the Gateway to the Barrier reef, So if You like to Dive or to see the Reef why not take an Excursion with friends or family by boat about 1 hour promises truely Breathtaking Coral and colourful fish and Underwater Fauna.
If your Looking for a beach Cairns is a no no it is Mostly salt marsh, Yes and it does have a lot of salt water crocs so becareful when walking on the outskirts of the city at Night.
Having said that the fireflys are very Wonderful to see.
If You Like Opals Why Not by Her a Ring or pendant Lots of tax free Shops in the city.
I got PADI certified through Down Under Dive. It cost app. US$250. This included 2 days of pool excercises along with classroom lecture/exams. Then you get 2 days out on the reef to do your open water certification dives and look around. In between diving you can snorkel around which I recommend, the reef is just as amazing at the surface as it is down under. They also gave me a free day out on the reef to dive and snorkel but I got sick and couldn't use it.
You can sign up for one of many daily cruises to the Great Barrier Reef for a day of snorkelling and scuba diving. The water is so clear and the clarity is good, you can see the bright colours of the reefs and fishes like an aquarium. If you don't have a scuba diving license, you can opt for an introductory 1/2 hr or 1 hr scuba diving session led by experienced dive master. They will literally drag everyone along in the waters, so it is pretty safe. This is my first experience with scuba diving, and I love every minute of it.
If you are in Cairns you must do a dive on the Great Barrier Reef. It's not the best diving in the world but not bad.
I went with a company called Cairns Reef Dive who I was very impressed with. The boat was good as were the dive guides on board. Lunch was tasty and they provided fresh fruit on the way back which tastes great after breathing compressed air for an hour and a half.