From Melbourne visit Philip Island to see the penguins & don't spend all the time staring out to sea because only a few come in from sea, most come out of the burrows behind you. We were advised incorrectly.
Great Ocean Road & the 3 sisters from Melbourne.
Also from Melbourne walking on Wilsons promontory.
Also in Melbourne the seaside suburb of St Kilda's Beach, the markets & a hop on hop off tour of the city visiting the different sites.
Haven't been to Adelaide however Kangaroo Island might be worth a visit. Not sure how far the old mining town of Cooper Pedy, where people live in subterrean homes is, but I reckon it's worth seeing if it ain't too far. about 6 years ago
Melbourne is my favourite Australian city - I love it for the food, shopping, walking around the city and the best customer service in the world.
Walking the streets of Melbourne for colourful cultural experiences, you will find laneways of artworks and great little cafes/wine bars.
Adelaide is also a lovely city - but a lot quieter than Melbourne or Sydney. Expect a lot of gardens and churches. Travel up to the Adelaide Hills for some "German" style villages including little restaurants and art galleries and then keep going until you reach the Barossa Valley (beyond the Clare Valley). about 6 years ago
At this time of year (mid-Summer), the beaches are great! Don't forget to wear *lots* of sunscreen as it's very burny, even on overcast days. Even just grabbing fish and chips for dinner and watching the sunset over the sea (in Adelaide) is beautiful.
Also, have a look at the other questions people have asked about Adelaide - you'll find lots of people with advice on 'interesting things to do on the weekend/for a week'. http://answers.worldnomads.com/search?q=adelaide about 6 years ago