A few comments (a lot of which I've copied from another similar thread I posted it in recently):

Great Ocean Road
The drive MEL-ADL via the Great Ocean Road is definitely worth taking, especially if you like food and wine in addition to walking, driving and awesome scenery.

Aside from the usual touristy (but beautiful) things which I'm sure you're familiar with (twelve apostles, london bridge, loch ard gorge, etc.) and given you mentioned you would like to do some walking, I suggest you check out the Great Ocean Walk (greatoceanwalk.com.au). GOW which stretches 104 km from Apollo Bay to the 12 Apostles, roughly following the Great Ocean Road, via the Otways, sections of board walk and beach walking. I did a small part of it (Blanket Bay to Cape Otway lighthouse) in January a couple of years ago and it was great - I'd love to do more of it. There are numerous spots you can start and finish. There's also a shuttle service if you want to walk one way and be driven back, not to mention numerous accommodation options, both on and off the walk.

If you like food, one option to stay is Port Fairy, a small old fishing village about 5-6 hours drive from MEL on the Great Ocean Rd. Port Fairy has quite a few good restaurants. There is currently a special on accommodation in Port Fairy via spreets.com.au until midnight tonight - $199 for two nights (which is probably too long for you, but an option) at The Victoria Apartments, including a $50 meal credit. Centrally located.

If you like wine, you can drive through a number of wine regions on the way from Melbourne to Adelaide (primarily in South Australia). Let me know if you'd like more details.

If you wanted to, you could definitely leave after your conference finishes at 4pm - it's about 2-3 hours drive to the likes of nice seaside towns like Lorne and Apollo Bay on the Great Ocean Road, or smaller towns like Aireys Inlet. It should still be light when you arrive. Plenty of accommodation options in Lorne and Apollo Bay. Just be aware that if you are leaving on Friday night in December, traffic is likely to be heavy, given school holidays and summer.

Kangaroo Island
I spent three nights there a couple of years ago, with two mates. We flew to Adelaide, picked up a hire car and took the ferry over. Having done that, I can see positives and negatives of both independent and a tour. The island is deceptively large, with attractions spread from one end to the other, so we ended up doing a lot more travelling than we thought we would (we stayed on the northern coast in the centre of the island), but we did have the freedom to do the things we wanted to at the time we wanted. A tour would reduce the flexibility, but remove some of the hassle and increase the social aspect. If you do decide to go it alone, be aware that most of the roads on the island are gravel (the only sealed road is the road running the length of the island east to west) and most hire car companies do not allow to take their mainland cars on the ferry or drive them on gravel roads (two that do are Thrifty and Europcar. Hertz, Avis and Budget do not). There are a couple of hire car companies on the island (Hertz is one). The ferry over is not cheap - approx. AU$92 each way + about the same for a car. There are some fares that are about half that, but limited. See sealink.com.au. Be aware that the drive from ADL to the ferry at Cape Jervis is about a 1.75-2 hour drive. Flying is also not cheap. There are numerous accommodation options on the island - the best pace to stay depends on your budget, what you'd like to do and how much driving on dirt roads you're prepared to.

wotif.com is a great accommodation reference - note it's not always the cheapest or most comprehensive, but one of the best.