I second the Great Ocean Road idea! Torquay is lovely. Apollo Bay is also great, a bit further away but lovely with stuff to do, and it's a bit flat there (if I remember correctly). Or Ballarat/Bendigo? Not the most exciting of places, but lovely and quiet and there's stuff to do in both.

Enjoy your trip, wherever you end up going!

This is a great thread - I've also got SPD and want to go on the babymoon - I'll be watching closely for more ideas!