There is a lot to explore in Tasman National Park, including the Three Capes Walk (4 days and 3 nights). If you prefer to do a day walk, you can also do them individually (Cape Pillar: 9 hours, Cape Raoul: 5 hours and Cape Hauy: 4 hours).
Other than that, check out Port Arthur, Eaglehawk neck and Doo town. Bring a rod with you if you love fishing and you might get some Australian Salmons or squids.