The Yamagata itinerary would be three days long, but more (or less) days could be added if you want to change things around.
The first day would include your arrival and a traditional boat ride with kotatsu. Kotatsu are like a table with a blanket and heater under them, they’re really popular in Japanese winters and I think they’re fun to try out. You could spend the night at Ginzan Onsen for the famous night views in the snow, and try out a Japanese inn, ryokan. If you don’t want to spend the money on a ryokan, rather stay at a cheaper hotel, we can change that around and include your visit to Ginzan during the day instead.
The second day would be a day trip to a traditional town for some cultural experiences and visiting some of the sites that appear in the movie Departures, and the third could have a festival and temple visit before you head back to Tokyo.
Of course, this is just a first draft without knowing much about your group and interests, but if you’d be more interested in other things we could easily change them around.