I drove the Great Ocean Road from Melbourne in a day and wished I had skipped 3/4 of it. I do not particularly fancy driving, so I have some bias.
I spent 8 hours heading out along the road with many stops at the scenic goodies at the end, then 3 hours back to Melbourne. It takes 1-1.5 hours from Melbourne to pass Geelong and begin the Great Ocean Road, B100.
There are four distinctly-themed stretches.
1. Torquay to Angelsea: surfer paradise, centered on Bells Beach. Quicksilver and Rip Curl are based in Torquay but their flagship stores are modest. So too the views.
2. Angelsea to Apollo Bay: the twisting cliff-hugging roads of legend. I liked it but the heavy traffic and the next section left me nonplussed.
3. Apollo Bay to Port Campbell: this inland section sucks. One slow car diddling along and everything grinds to a halt. Mostly farm country, some forests.
4. The Twelve Apostles (actually before Port Campbell) and associated rock formations all the way to the Bay of Islands. Awesome.
I am a completest so would always drive the whole route despite my advice, but my best memories are out at Twelve Apostles and Bay of Islands which I raced to see before sunset. Port Campbell is a nice town and quieter than the earlier tourist towns. Especially for a day trip, consider taking the Princess Highway (A1) from Geelong all the way down to the junction for Port Campbell and enjoy the best goodies first. If you have surplus time, consider working back to get the coastal curves starting from Apollo Bay.