Just back from an Adelaide to Perth trip as part of my desire to complete a lap of Australia (not helpful to you, I know!) But I did Sydney – Melbourne – Adelaide in October 2015, specifically to go to Phillip Island for the motogp race. The drive south from Sydney isn’t the most scenic with not a great deal of wildlife. Phillip Island does have the penguin parade which I’ve enjoyed both times I’ve been there.
Melbourne to Adelaide has two options, either following the Great Ocean Road or heading inland and going via Halls Gap to see the Grampians. I’m fortunate enough to have done both but if you time it well with good weather, the Great Ocean Road would win with some amazing scenery. Depending on how long you have, you can also go to Kangaroo Island, which has some stunning beaches interesting drives on some unsealed roads (check your rental agreement first, obviously!) and good for wildlife spotting- the only place of seen an echidna in the wild and I’ve been trying for ages.
I also know what you mean about the drop fees, they are buttock clenchingly expensive!
Whatever trip you do it will be amazing, it’s Australia, it’s impossible to not enjoy it.