I lived there for a couple years.
If you live in the West, you'll probably find the skiing a little underwhelming. The lack of attitude in the parks is refreshing though.
Places to see? I think Kansai (Osaka, Kyoto, Kobe, Nara) is where it's at. So much fun to be had there. Hokkaido is your best bet for skiing/hiking though. The Sapporo brewery in Sapporo is definitely worth a visit. There are loads of other places, of course.
Things to do? Other than the general seeing things and walking around, Japan is a great place to eat and drink. So much good food, and I'm a big fan of the beer as well.