Fraser Island is brilliant for a couple of days - biggest sand island in the world - you can camp over there and there's no speed limit on the beach (four wheels are best here not two !!). It's just over halfway between Sydney and Brisbane.
Then there's the Gun Barrel Highway - dirt track from Adelaide to Perth running through the Nullarbor Plain. It's supposed to be an amazing adventure. You do it on dirtbikes - I think there are places you can hire machines at either end, check the Net. (see the link - although I've had nothing to do with these guys so can't vouch for their services, but it gives you an idea -
http://www.hctravel.com/austral.htm). Have fun you lucky, lucky b'stard.
Jiminy