Asterix Perth is great for a stopover and a few days driving south to the wine region of Margaret River. You can go snorkeling with whale sharks at the right time of year at Ningaloo Reef - but it's a long way even from Perth, which is itself a long way from anywhere.
Without prejudice, southwest WA probably has the best beaches in the country. It is a great place to spend time, but I think you can have a lot of the same sorts of things but a bit more action-packed over in Tasmania. That said I will be camping out on West Cape Howe on Saturday eve, and it'll be stunning.
@HomeSteak Honestly, nothing wrong with Sydney and Melbourne but I can't understand why anyone would fly here to tour them.
These would be my 2 week Australian itineraries …
Tasmania circuit … something like Hobart, MONA, wine and cheese, hike and camp Cape Freycinet, Port Arthur, Bruny Island and then maybe jam in a Cradle Mountain hike would be a pretty good combo for a week or two … this one's for the summer months. Head back to Melbourne or drive the Great Ocean Road a bit.
Queensland city to the tropics tour … Brisbane, Glass House Mountains, Atherton tablelands, Hinchinbrook Island (especially if you like hiking, the Thorsborne track is amazing but you've gotta book early), Cairns, dive the Great Barrier Reef, hit up the Daintree. That is a lot of beautiful stuff.