Winter festivals in Japan (illumination?)

My wife and I want to see Japan, we live in Thailand and she wants to see a "wonderland of winter". We will have a maximum of 15 days to visit Japan, it will be a budget trip.

1. What will be the best Christmas experience? Christmas decorations in the city and a festive atmosphere of Osaka. Kyoto or Tokyo? I know ... it's not something that Japan really celebrates!

2. All the beautiful winter places seem to be in the north. then my route will fly to Osaka from BKK. Then Kyoto, a few days later, flies to Sendai. Then, the bus trips ... I guess ... to Ginzan Onsen, then to Ouchi juku and then to the Japanese Alps in Gifu prefecture. Then slowly to Osaka through Kyoto.

Has anyone made similar winter trips? For this trip, are buses available? Do you train in some cases?

General comments welcome. Thank you very much.