Living in Hobart - my opinon is thus...
Saturday DEFINATELY do the boat cruise - as anyone who did OzFest this year will tell you, its amazing.
Sunday - thats gonna be a LONG day, but an interesting one.
Monday -The tahune airwalk is lovely and if you are heading that way you may as well go to Hastings caves (its only another 40 minutes or so down the road). The other option is to hire a car and just drive up the east coast and see how far you get! What do you like doing? I can then offer other suggestions.
Tueday - if you are on the midday flight, take a longish walk to Pigeon Hole cafe in West hobart for the best beans on toast imaginable.