The car rental company will not allow you to rent a car in Melbourne and take the ferry to Tasmania. Instead, you have to rent the car in Tasmania and there are a number of car rental companies in Devonport.
For sightseeing, you can consider the following:-
Bicheno......to see or even catch the penguins in the evening near the blowhole.....free of charge and please bring along the torch
Freycinet National Park Walk.....to see the beautiful Wineglass Bay in the distance
Cradle mountain
Tasmania Devil Park.......see the crazy Tasmania devils chasing each other
Russell Falls
Please drive carefully ......beware of rough and winding roads in some areas e.g. Queenstown.