Sounds like a great trip.

Mt Field is great and really close. The Russell Falls are lovely and then drive up to Lake Dobson ( I think that is right).There is a nice walk around the lake and if you have time and the weather is kind, walk up to the Tarn Shelf and wander along the walking tracks up there. The 3 different elevations mean that you will have a really varied day of walking

Port Arthur is interesting and a little bit of history. Near there, there are some great coastal day walks which I LOVE. From memory, it is Fortescue Bay. I will dig out my old Day Hike Guide and get some more specifics for you.

I don;t know anything about Bruny Island. My memory is that it is a long way for a day trip. The Haartz (?sp) Mountains are a popular day trip in our family. If the weather is bad ( and it often is- you don;t get rainforest for free!) Mt Wellington on Hobart's doorstep has a myriad of lovely rainforest trails and it doesn't matter if the sun isn't shining.

I will try to get some more details and get back to you.