Australia is as big as the contiguous US (ie. not including Alaska and Hawaii). It is hotter than you probably have experienced, flatter (the continent has some of the oldest land on earth) and drier. Melbourne to Sydney is not a morning's car trip - although it CAN be done in a day (if you really push it). So, you will probably only see one small part of Australia. I recently spent over a month seeing south east corner of SA including the Eyre Peninsula and the Flinders Ranges. Which small part are you interested in seeing at the moment? I'd guess the reef and the rock (and a few kangaroos, and maybe a koala or two). But only seeing tropical Queensland and Ularu in the Northern Territory misses an awful lot of Australia.
We have the oldest continuous society on earth (the Aboriginals came to Australia 40,000 or so years ago - some archeologists are finding dates around 60,000 - there is a world heritage area at Lake Mungo where the world's oldest known human burial and cremation occurred, and where the oldest set of human footprints have been uncovered). There is also the earliest known depiction of a human on the Burrup Peninsula. There are stromatolites in the Shark Bay world heritage area. There are forests of enormous Mountain Ash (the largest tree ever recorded was a Mountain Ash that was cut down over a century ago to find out how big it was) in the Victorian High country. The dinosaur stampede at Riversleigh (another world heritage area) in Queensland. There are coral reefs along the west coast as well as the east coast - I'll never forget visiting Ningaloo and seeing the sunset glowing through the coral (must have been low tide because the coral must have been a metre out of the water) from the beach. The Flinders Ranges in South Australia, and the Tasmanian Wilderness Area. Everyone I know raves about Cape Le Grande National Park. There are numerous cliff/gorge areas - Blue Mountains, Bungle Bungles (I think they have another name now), Grampians (also renamed), Warrenbungles (although when I went there recently there had been fires), Canarvon Gorge, The Olgas, Hamersley Gorge... again, these are all over the place.
If you like cute furry animals, koalas live more in the south east than anywhere. If you are on the roads for any length of time, away from the cities and the coast, you will see more kangaroos than you care to (they are mobile pogo sticks, and jump at cars at sunset) - and you will probably see a few emus. Fairy penguins can be seen at Phillip Island in Victoria (there are colonies elsewhere). Platypus are difficult to spot, although I have seen them on the inland rivers and coastal rivers in Eastern Australia - Healesville Sanctuary outside Melbourne is probably the best place to see them. Possums are often visible in cities.
There are winery areas in WA (Margaret River), SA (Barrossa Valley, Clare Valley...), Victoria (Yarra Valley, Rutherglen...), NSW (Hunter valley, ACT regional wineries...).
The Great Ocean Road is fantastic, and there is a road over the ocean near Wollongong. The South Gippsland Highway and Princes Highway in Gippsland/south coast NSW is nice - both roads are awful further west - and wandering that area can include Wilsons Prom National Park and Grand Ridge Road including a visit to Tarra-Bulga National Park.
Staying in National Parks can be quite cheap (although Wilsons Prom is expensive).
Melbourne has arguably the cheapest and most amazing food scene - Brunswick, Footscray and Richmond are good places for a variety of cheap restaurants - as many migrants settled there. The Australian War Memorial in Canberra was voted Australia's top tourist attraction last year (yes - the rock and the reef both missed out on top spot). Sydney has the harbour, but you just have to see how beautiful Perth and the Swan River are from Kings Park, and Adelaide and Brisbane are also worth visiting. I haven't been to Darwin.
Outside the capital cities, towns get small very quickly - particularly in the west. WA takes up a bit less than half of Australia, and if you drive more than an hour and a half from Perth, the largest town in all that area is just under 30,000. The east coast has bigger towns, but there are only 25 million people in Australia, and we are about the same size as the contiguous US. This means that everything that is not local is very expensive - fuel, food..., secondary roads get poor, there aren't many services available, and every town is a long distance from the next one.