After travelling around Japan a bit, we've started to compile things that we should bring with us when we leave our house. Check this page often, because we'll keep adding things as the need arises!
Clothes: Pack accordingly!! Summers are hot and sticky, winters can be fairly cold. Socks: Bring plenty of 'em and CLEAN!! You can't go into a Japanese house with shoes, and barefoot is considered rude.
Little Packs of Kleenex: A lot of the public bathrooms don't supply toilet paper. Bring a pack or two of Kleenex with you wherever you go. Hand Sanitizer: Some of the bathrooms we have run into had sinks but no soap. So bring something to clean your hands with, just in case.
Good Walking Shoes: This is obvious, if you're planning to do a lot of walking. And you will. Yen: Don't forget the yen!! It's cheaper to exchange in the Japanese airport than in the U.S., we have found. Most places don't take credit cards, so yen is crucial.
A fan: Of course, you can buy them while in Japan, which would make a great souvineir anyway. But you'll need one if you're walking aroud a lot in the summer time. An umbrella: Especially if you're coming to Japan in the rainy season, which is June-July (and the typhoon season which is in September.)
PASSPORT!!! Don't forget it!! Or you'll never get in!! Towel: This isn't entirely necessary, but most of the towels in Japan are pretty small. If you want a nice beach-sized one, bring it.