The best thing I did in Australia without any shadow of a doubt was scuba dive on the Great Barrier Reef.... Amazing, second best and not often mentioned was climbing King's Canyon for sunrise... beautiful
I agree with Moog.You have to dive at the Barrier Reef and climb Kings Canynon (tough!), but you also HAVE to go to Fraser Island, Whitehaven Beach at the Whitsundays, hold a koala, go to Steve Irwins zoo, visit the Castlemaine brewery, go to Byron Bay, Noosa, Perth, Mission Beach, Surfers Paradise, Alice Springs, Brisbane, Adelaide etc etc etc etc. God, why did I come home!