I just spent a couple of years driving all over Australia so might be able to help
Really it all depends on how long are you going for, and what you're after. If you just want to party then... err... stay at home - you get just as drunk but the beer's better here
You're best getting out of the cities if you want to really see something of Australia. Have a day in Sydney and Melbourne of course, but once you've seen the sights you'll find the cities, zoos, aquariums, theme parks etc., are exactly like European cities, zoos, aquariums and theme parks so I really wouldn't bother spending too much time there.
And unless you are there for the partying don't bother with Bondi Beach either. It's absolutely packed and there are miles of better and quieter beaches everywhere along the coast.
From Sydney the Blue Mountains are an easy drive and worth seeing. Quite touristy but still pretty. There's a fantastic YHA hostel in Katoomba too.
Then I'd suggest heading back and taking the coastal road from Sydney to Melbourne rather than the direct road. That whole coastline is great: you can see dolphins in Jervis Bay, stingrays in Narooma, Eden, Pambula, Lakes Entrance etc.
There's also a couple of rainforest bits if you wanted to see them on the way and it'll take you straight to Philip Island too.
From Melbourne, definitely do the Great Ocean Road as planned; it's really a great drive (we did it twice

).
The Grampians are a couple of hours drive north west from Melbourne and are really pretty. You get a good idea of the size of the country looking out from those hills. Stop at Ballarat on the way to the Grampians too. It's an old gold mining town so it's a bit of history - Oz style.
If you're going for longer than a few weeks, I have plenty more
Oh, and in Frankston, you'll find that the
Mullet is alive and well
