If you were given an extra week in Japan at the end of your trip, where would you go?

i'm planning a month-long trip to Japan from mid jan-mid feb, and most of it is along the golden route. i was thinking of trying to go somewhere either north or south for the last 5-7 days of the trip. curious if anyone has any off-the-beaten-path recs of underrated or under-visited locations that would be with a short flight, etc! thanks!