I have, of course, been talking to people about it -- a friend in Tokyo, pinged another friend in Yokohama, and I've been picking the brains of all the people I know here. And, of course, there's my map. Also picked up the Lonely Planet guide -- not so much as a reference as something to give me ideas of places to go.
So, originally I was thinking (along with stuff in the Tokyo area) of hitting Nagoya, Takayama (Denver's sister city), and Chuubu-Sangaku National Park (the "Japanese Alps"). But, I've decided that would be better for later in the year (September, after the WorldCon), because transportation (in the park in particular) would be pretty iffy in March, since it's a month or two out of season. So, I've got that trip more or less figured out, at least in general terms.
Which leaves the north route for this trip for a week, at most a week and a half (the rest of the trip being spent nearer to Tokyo). I already wanted to go to Hokkaido, and jariten has convinced me that I should swing through Akita, ancestral homeland of Hachiko, the faithful dog of Shibuya, it being a place with some mountains and stuff, making it unique in all of Japan (or not -- yeah, yeah, dude, I know. Onsen).
I'd sorta been thinking about climbing Mt. Fuji. But doing it in March, even with reasonable winter mountaineering experience, gear, and all, is probably just plain impractical. It'd be a rather major logistical challenge, alas, not to mention moderately dangerous, no matter how cool it would be to watch dawn at the summit on my birthday or something. So, I've been backing off of that, and thinking maybe I'll try in September, which is just outside of peak season, making it about perfect, since there would still be some minimal support, but no crowds.
So. There it is.