Sorry, I have not seen this message early enough.
If you still read this answer, I just would like to underline that National Parks are not very practical in winter, especially in Hokkaido: journeys can be difficult, and many interesting features that require walking are usually not reachable.
From Sapporo, I would suggest visiting Hakodate - there is a good guide on this city on this web site. It takes about 3h15 by train to travel from Sapporo to Hakodate. There are also some good onsen town around Hakodate.
For onsen, there is also Niseko, which is a famous ski resort. There are many buses from Sapporo to Niseko, but I do not know if there is good cross country ski there.
Finally, you probably would not need advance reservation, as this is likely to be before the beginning of the busy period.
sanji
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