Getting to islands can be tricky, but the two BIG islands of Tasmania are destinations in their own right: Flinders and King.
Bruny Island is lovely; as it is an incredibly popular destination make sure to plan ahead to check on ferry and destination details. I’d also suggest heading to Triabunna and visiting Maria Island on foot or with a bike.
The convict heritage of Sarah Island can be visited on a river cruise of the Gordon River in Strahan. You’ll also see the Southern Hemisphere rainforests of the Southwest Rivers.