The Great Ocean Road has some amazing scenery, but I fear that it's dead as a driving road. I sincerely hope your expereinces are different but of the three or four times we've given it a go the experience has been soured by grey nomads doing 30 in 100 zones (that is not an exaggeration), tourists forgetting we drive on the left, and painfully low speed limits along with overbearing enforcement, presumably as a bandaid for the previous two and the people trying to get around them. As I said I hope you guys have a blast, and others may love the place, but thus far my experiences haven't been great (no pun intended).
See if anything is happening in Geelong, there's usually some kind of car-related event going on there, and you'll be passing through anyway if you're going to the GOR. Generally more around classic cars but big car culture down that way.
I hear that the Great Alpine Road is nice in summer. I've only done it in winter but it'd be no less spectacular I'm sure in summer, though perhaps someone who has done it can confirm. A bit of a hike if that worries you.
As already mentioned the roads around the Dandenongs are great if you can get them on a quiet day, you can have fun on them without doing anything illegal. They are similar in a lot of ways to Glorious/Nebo/Mee but the surrounding countryside is even nicer IMHO. And it's winery country. Look up places like Reefton, Marysville, Yarra Glen, Kinglake, Healesville as mentioned etc.
I also don't mind up some of the less-travelled roads between say Werribee and Bacchus Marsh, Gisbourne, Ballan etc.