If you enjoy snowboarding or skiing, I highly highly recommend hitting the slopes there on Hokkaido -- truly some of the best snow & biggest runs I've ever had the pleasure of riding. Better than Colorado, Utah, Alaska, or anywhere else I've visited.
Also second the recommendation of waiting until the ice festival in February. It's #3 (after Mt Fuji & Hokkaido snowboarding) on my bucket list of things to do while I'm living here in Japan.