Are you saying you still want to do both Cradle Mountain and Maria Island? In 4-5 days; flying in to Launceston, the first two nights at Cradle Mountain to get a full day of walking or visit Mole Creek, then 5 hours drive to Triabunna for the Maria Island ferry (I would personally do a day travel via Bicheno where there are red rocks and Freycinet and stay overnight at Swansea); followed by a day / overnight on MI, means you have one day left to explore the Tasman Peninsula on the way to Hobart airport. You can't really fit any more in.
However, there are plenty of other nature / wildlife / outdoorsy things along the north coast too. To the far west is the Tarkine (~3 hours from Cradle Mountain) which is (currently) pristine rainforest - search for the Tarkine Drive and Arthur River but check car rental details for unsealed roads and explicit exclusions (e.g. the 'western explorer'). Between Stanley (The Nut) and Rocky Cape area there is geology and some very nice beaches. Latrobe is platypus central. narawntapu is full of kangaroos and wallabies. Low Head and Penguin have penguins (who'd have thought?). The Tamar Valley inland of these has excellent wineries and dairy. Further east becomes hilly and forested again, with dedicated mountain bike tracks, to the low-rise wukalina-Mt William which has deserted beaches and kangaroos.