I spent last Christmas at Fleurieu Peninsula, SA. It is a great place for family holiday. If you like wine, you would enjoy going around wine-tasting in the area. As I have young children, I tend to stick with activities suited to them.
Please visit my photo blog if you like to see some pictures of the place.
http://fleurieupeninsula.blogspot.com/
It's about an hour drive from Adelaide, to enjoy the place, I think you really need a car.
I live in Melbourne, so we drove along the coast and visited a few places on the way. If you are interested, you can see some photo blogs here.
http://sa-holiday-jan2009.blogspot.com/