I changed my ticket to fly to HBA instead of CNS. Thanks all for the suggestions.

I'm thinking of 5 nights but could add 1 or 2 more depending on what kind of trips outside of Hobart I would do.

I see that there are a lot of coach tours, to Wineglass Bay, Port Arthur, Bruny Island and the Freycinet National Park. These are full day tours but I may be able to avoid hiring a car, though car hire prices aren't that bad -- I've driven plenty of times on the right side of the road.

The only one where I might be interested in driving would be Port Arthur I believe, maybe overnight there and try to get out to some of the coastal areas to the south and SW of Port Arthur instead of just visit mainly Port Arthur on the day trips. I think these are more centered around the historic nature of the settlement, without any chance to look the the scenery which is relatively near Port Arthur.