The Great Ocean Road is very stunning
The Great Ocean Road is stunning for a Mid-western boy like me. We did it as a day trip only going as far as the 12 Apostles. I was told there wasn't much in the terms of scenary beyond the 12 Apostles.
Melboure has alot to offer in terms of city life. The only mid-western city that even comes close would be Chicago.
I would suggest seeing the penguins on Phillips Island. I would NOT suggest using Gowest tours as they will stick you in an uncomfortable van with no bathrooms. They also hurry you off to get back on the bus much too early.
Sounds like you plan on driving yourself so that isn't much of an issue.