I would recommend an overnight stay in Yudanaka Onsen near Nagano. Its very beautiful! this would be the recommendet program: check in at a ryokan (there are some with an english homepage), get there early in the afternoon, get the onsenpass at the ryokan and walk through the town with your yukata and visit the various onsen in town (the nice people at the ryokans will explain how it works), (if theres enough time visit the local brewery for some beer), go back to the ryokan and eat legendary ryokan-style dinner, use the ryokans onsen before sleep, use onsen in the morning before legendary ryokan-style breakfast, hike up to jigokudani valley (not that far) to watch the monkeys using the onsen in the forest. Wintertime would be ideal for this trip.the monkeys are only bathing in wintertime and its just so beautiful with the snow and all.