Hi Kemjak, where are you coming from? I would highly recommend the South East of SA, from Goolwa its a 4 to 5 hr drive to Mt Gambier (440kms), in the Mount you have the Blue Lake region, the sinkholes, Tantanoola Caves, Nelson, Port MacDonnell all within 30mins. An hour away you have Penola, Naracoorte (Naracoorte Caves) and the Coonawarra wine region.
An hour away down the coast you have Beachport and Robe for magnificent coastal scenery.
You could easily fill 5 days down there.
Flinders Ranges is accessible from Goolwa but its about another 100 to 200kms further than the south east trip but does offer magnificent scenery.