See, my guide book has already mislead me. I didn't want to sound like the Aussie I am not, but G'day was supposed to be a friendly way to say hello, and I read to say mate to the man you were addressing and bloke if addressing many. In Hawaii one must be careful trying to blend in when it is obvious you don't. I have a tendency to pick up an accent after a few days and that can be offensive, especially when it is obviously wrong. Thanks for the suggestions.
You can probably get away with calling people "mate". You can really only get away with "g'day mate" if you've got a broad ocker accent like Paul Hogan, though, it has become a caricature of Australia more than something people actually say. "Bloke" is more of something you say about other people... "those blokes over there". I wouldn't ever say "hey blokes!".
Oh, if you have one of those bags you put around your waist, do not call it a fanny pack. "Fanny" over here refers to, well, naughty bits. That's probably the #1 word-related embarrassment for Americans here.
If your family is in to sports, July-August is a great time to catch a live game of Australian Rules Football, it's a pretty unique sport and fun to watch even if you don't know what's going on due to the fast pace and athleticism of it (or so I have been told by all the overseas visitors I've dragged along to games).
But with a limited time available, I'd say the first priority should be to spend a few days up at Port Douglas or Cairns and use that as a base to visit the Great Barrier Reef and the Daintree Rainforest and see some native animals. Fly in to Sydney, see the harbour and the Opera House and whatever else, then up to Port Douglas. After that you could go out to Alice Springs for Uluru/Ayers Rock and maybe other desert attractions, down to Melbourne, or down to Tasmania- Tasmania's a beautiful and compact place, you can see wildlife sanctuaries, glorious national parks (mountains, forests, rivers, caves, beaches- whatever takes your fancy) and historical remains of the convict era all within 1 day if you wanted to. Because everything is so close together and there's so much unique stuff there, like the Port Arthur convict settlement and Tasmanian Devil sanctuaries, the Museum of Old and New Art (MONA) and all the national parks, it'd be great no matter what your family is interested in.
The only trouble with Tasmania in July-August is it will be cold. Not snowing, but still really cold (everything in Australia south of Port Douglas will be cold at that time of year, but Tasmania especially).