Presume you are thinking of turning off the Western Highway at Keith, Jennifer in Hobart.
If I was doing it I would go Robe first night (similar sort of place, atmosphere wise to Lorne except it isn't on the open sea). You turn off at Tailem Bend for that. Then Lorne the following night.
You can turn off at either Kaniva or Ararat to get to Warnabool as well. Kaniva is probably the most direct but I wouldn't recommend if you are trying to make good time. The wander through the Western district of Victoria slows you up a bit.
If you want to make good time, probably Ararat is the best. Bit further but you are on the Western Highway for much of the time. It is a pretty good run down from Keith as well but either way with a few pit stops it will take about 10 hours.
The great ocean road is good but perhaps a little over rated. You get a far more inspiring view of an ocean from the eyre Highway on the way to Western Australia. The winding road actually detracts from the spectacle a bit if you are driving and, heading east as you will be, you will be on the side away from the ocean.
The Barossa Valley is pretty standard fair for a day out of Adelaide but I like your other idea better. If you go to
Goolwa you can see what they call the barrage. It seperates the salt and fresh water in the MUrray. It is in a pretty sad state at the moment with the drought but when the Murray is in full flow it is a brilliant thing to have a look at. Well brilliant might be overstating it a bit. Fascinating never theless.
There is also Hindmarsh Island which has a controversial bridge project.