Went back in 2003, stayed on the east coast - Sydney, Gold Coast, Brisbane, GBR.
Definitely Great Barrier Reef, although my wife needed a barf bag on the trip out and back. It's a longer trip than you might think.
We saw La Boheme at the Opera House. I'd recommend going whatever is playing there. Sydney was nice, but other than Bondi beach, if seemed like a warmer Toronto. Good Asian fusion food, wine was ridiculously cheap when we went - should still be a bargain.
If you like wine, find a broker. It's worth stocking up. We shipped back about AU$700 of wine, and even with cost of shipping, saved a ton over buying same in US. They'll even set you up with a tasting to map your palate for a few bucks.
Good zoo in Brisbane, wife got to hold a koala, didn't catch Chlamydia. Gold Coast has nice beaches and surfing/belly boards, but mostly touristy, however, if you're driving from Sydney to Cairns, it's a good place to stop. Probably better to fly though - intracountry flights were cheap when we went.
We also stopped in Auckland on way there. It cut flight time from 17+ hrs to 14. Auckland is a nice clean city, great harbor, great food and a world-class aquarium worth visiting.
I would've loved to go to Tasmania, but time didn't allow. If I ever went back, I'd definitely spend time there.
Keep in mind that tf you're driving around, they use speed cameras, so a month or so after you get back home, you might find the rental car company is sending you a bill for tickets
