Enjoying the hot springs (Onsen) is definitely a recommendation, but how about some indoor activities such as visiting the trapoline park, experiencing Japanese culture with workshops, shopping at the famous outdoor brand stores (Snow Peak, Northface, Patagonia, and etc.), and enjoying a relaxing time in lovely cafes and restaurants?
For details, please refer to these pages.
Onsen
Shopping
Hakuba Japanses Workshops
Other activities
Restaurants and cafes