Best itinerary for 4 between Dec 18 - Jan 3?

Hi! There are just 2 must-see and do things on our list so far. We'd really like to go to Niseko for at least 3 days as the kids want to try to ski and snowboard. And being theme park fanatics, another visit to Fuji Q is on their list. Apart from that we're very open to recommendations on where to go during this period. Visiting the most festive parts of Japan around the holiday season would be great.

The plan is to enter via Tokyo and go to various destinations from there.

What would be the best itinerary to maximize our period of stay and enjoying the best of Japan during Christmas and New Year?

Thank you kindly.

Well. The new years things are pretty much closed. By things i mean shopping and restaurants. So you might want to spend your time in the comfort of a resort instead of a city. Since in a resort... everything is provided. So you might want to use that time (31 Dec to 2 Jan) to visit a ski resort).

Another option is to visit an onsen resort. Since the climate would be colder, dipping oneself in a hot spring is the ideal Japanese way to warm the body. Ginzan, Zao, and Nagano are mountainous hot spring resorts that are sure to let you enjoy both snow and warmth. Plus great local food if you pick a good ryokan. Nagano being the location where the winter Olympics was once held could be another optional location for skiing. Also good is that it is nearer to Tokyo and more accessible than Niseko.

Since the new year is when couples and families go out, it might be best to avoid any theme parks during this time.

If you want to avoid the cold, you should visit places like Kyushu. Kyoto, Tokyo, and Hiroshima may or may not snow but temperatures would not be as cold as Hokkaido (worst is if transport is down due to heavy snowfall). Also it is along the shinkansen line so travel is convenient.

My recommendation is start your time in Hokkaido.
Plane flight to Itami Airport.
Visit USJ, Kyoto, the rest of Kansai and Hiroshima.
Train to Tokyo.
Train to a ski/onsen resort for the new years.
Fly out.

A few things are closed at new year, but there is still an awful lot of places open.