Melbourne to the Twelve Apostles, with rainforest and surf beaches in between.
The Great Ocean Road is usually sold as a day trip from Melbourne, which is how most people end up seeing the Twelve Apostles at the same time as four hundred other people and driving back in the dark. It's a much better three-day trip.
Split it: surf coast and Bells Beach on the first day, rainforest and the Otways on the second, limestone stacks on the third. Drive west, so you're on the ocean side of the road the whole way.
One car, three nights, and self-catering keeps this cheap. The one thing to fix in advance is who's driving which leg — the road is winding enough that nobody wants all of it — and where you're sleeping each night, since Port Campbell has limited rooms.