St Arnaud is a small alpine village on the northern shore of Lake Rotoiti, where the road enters Nelson Lakes National Park. It is quiet, practical, and surrounded by beech forest and mountains; the lake is only a short walk from the village centre.
This is not a town of urban sights. Its value is that several excellent outdoor days start almost at the doorstep. You can walk the Lake Rotoiti Circuit, climb the Mount Robert Circuit, or begin longer tramps into Nelson Lakes. The DOC visitor centre is useful before you go, especially because the park is alpine and weather can turn hard in any season.
If you are passing between Nelson, Blenheim, and the West Coast, St Arnaud is the kind of stop that can easily become the best spare day of the drive.