Having just come back from down there I'd also recommend the car rental option, especially with the limited time you have - it'll let you get the most out of your trip. From the abundance of roadkill visible anywhere we went I think driving after dark is something to be done with caution though.

Not sure if I'd want to do a trip to Queenstown or Strahan and back to Hobart in a day, it would be a pretty long day and whilst the road is fun to drive along it's pretty curvy so takes it out of you after a while. But probably worth an overnighter if you can spare the time.

You could also get up Freycinet way on a day trip if you were keen, Wineglass Bay is spectacular. The Tahune Airwalk was fun too, though perhaps a bit of a drive if that's all you're going to see in that direction.

Around Hobart I agree the Harbourside areas are good to stroll around, and if you get there early enough on the 7th you can check out the Salamanca Markets too. Also good resturants in North Hobart on Elizabeth Street, a nice big collection of them to cover pretty much all tastes and not as pricey as the waterfront ones.

Hope that the weather's as mild for you as it was for me, if so it'll be a great weekend I'm sure!