Devonport is not a city to visit. It is a place of transit between mainland Australia & Tasmania. As a result, there isn't much to do in Devonport itself.
Don't stay here, just drive on through. Its a port city, and that's about it. Apparently there is a really good leather craftsman on the outskirts.
Visit him. I would recommend you drive straight to Sheffield after that.
Travelling from Devonport to many of the lovely towns in the surrounding areas is an easy drive on well maintained roads.
Around 40 minutes south west of Devonport is the lovely little town of Mole Creek. The town has a nice little pub with friendly staff and great food, and not far down the road are the Marakoopa and King Solomon caves, well worth a visit. Also in the area are many small businesses, such as local primary producers (apples, cherries, honey, vegetables etc.), and also many antique and bric-a-brac shops. There are also many B&Bs and farm stay accommodation places in the area.