Australia is an adventure in and of itself, what with all the shrimps on barbies and crocodiles. In addition to being home to people who say "Crikey!" without irony, Australia is also home to the Great Barrier Reef, the world's largest collection of coral reefs. The Great Barrier Reef encompasses 400 types of coral, 1,500 species of fish and 4,000 types of mollusk (you could say it's very molluscular). It's also home to endangered species like the dugong and large green turtle.
If you visit the Great Barrier Reef, you have plenty of options. Some tour companies use large boats to take groups out with educators to explain the different types of coral, environments and challenges that face the reef as a whole. You can also charter your own boat, go on a smaller boat with a guide or explore the reef by SCUBA diving or snorkeling. Small cruise ships also explore the reef, and allow passengers to take side trips to some islands in the area, including Magnetic Island, which many people just feel drawn to.
Before you go, you should know that the Great Barrier Reef is not only important as a natural site but also as a cultural site for native peoples of northeast Australia. It's also extremely fragile, so going with a knowledgeable guide will not only enrich your trip, but could also keep you from inadvertently harming the reef and being chased out of Australia by angry men named Bruce.