I live in Hokkaido and visit Sapporo 1-2 times a month. The weather has been pretty odd this year with it warming up about a month earlier than usual. That being said, since you'll be flying into Shin-chitose anyway, why not stop by? Not sure how big of a ski guy you are, but an entire week sounds a bit tiring.

Hokkaido in general has great eats and Sapporo especially has some pretty cool places to check out. Yes, it may be mushy out, but the underground walkways and subways make it (somewhat) easy to get around. I could easily spend a day just wandering around in the station!