So I am headed to Australia for two weeks. Arrive in Sydney and leave from Melbourne two weeks later.

Maybe some of our Australian friends here can give me some insight. Or perhaps others who have made the trip.

I have the first four days dedicated to Sydney then head somewhere else. What to do next?

I am debating on:
Cairns and the Great Barrier Reef
Tasmania and the Southern Lights / Wildlife / Landscape
Adelaide and Kangaroo Island
Hire a car, log on to booking.com, live day by day and just drive with no defined destination except Melbourne 10 days later.

Some other less promising ideas, Perth or Alice Springs although those ideas are less likely.

Suggestions?

Gear to take? The only question would be do I need my 500mm, and that will depend on which direction we head. I think yes in Tasmania, no in Cairns and maybe in Adelaide.