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Passport: Make sure it has at least six months validity and one empty page.

Visa: Many nationalities can enter Japan without needing to arrange a visa in advance, but it’s best to check ahead. See Do I Need A Visa For Japan for more details.
Flight tickets: Even in this day of electronic ticketing, it’s helpful to have a paper copy to refer to and show airport staff. If you are still researching flights to Japan, Skyscanner is a useful way to compare multiple airlines’ flight times and prices
Hotel reservations: Having a print out is helpful for hotel staff as well as for yourself. If the hotel provides an address in Japanese, this is useful for taxi drivers. Also, you’ll be asked to fill in your first night’s hotel address in your landing card before immigration, so a printout is handy. Book Japan hotels online at Booking.com and Agoda.com

Buy your Japan Rail Pass if you’re intending to travel around the country. You need to buy this before you arrive in Japan.

Travel insurance is vital for Japan and indeed anywhere in the world. Japanese health care is excellent but also expensive. Besides your health, you want to be protected against cancelled flights, missing baggage etc.

Credit Cards and Debit Cards: VISA and MasterCard are widely accepted in Japan – American Express not so much. Ensure you tell your bank before you leave your home country that your card may be used in Japan. Also bring an ATM debit card to withdraw Japanese yen from your checking account.

Make a copy of all your passport and credit card details: Use your cellphone to take photos of your passport photo page and the back and front of your credit cards. Email them to yourself in Gmail or other internet accessible email account. Now you have all the details and emergency numbers if you should lose your cards or passport.

Wifi and SIM Card access: It’s easy to get your own data-only SIM card in Japan and also find public free wifi in Japan. Read our in depth pages about Where to Buy SIM cards in Kyoto and Tokyo.

Japan Maps: There’s still no quicker way to get a sense of the big picture than looking at a hardcopy map. For Japan, the best commercially available maps are made by Periplus.

I hope you make a lot of photo's from us ^^

Hi I am currently on my japan trip!! (Tokyo)

In here japan, and usuing subway(metro), you can buy your train pass.
+)you can buy pass here in japan.
This train pass has many options by time. Like 24hr,48hr,72hr.
This pass is usable in most of the line except for the monorail :)
=> I think this is important because in japan, if you are changing lines, you have to check in and out the doors many time.
(I bought 72hr pass cause i am in japan for 3days. )
Also, since japan uses a lot of coins,
=> bring your <coin wallet> or some small bag to put coins!!

I bet this informations helped you!!!

If you plan on buying some t-shirts, you can reduce the amount you take with you. Like, going for 7 days, take 3-4 shirts, buy some there.

For luggage, something that keeps your stuff next to you. Maybe a backpack w/ tons of zippers, maybe a purse, or a coat with tons of pockets.

If you can read Japanese well, a pocket dictionary is optional. Tho I think a translator app is mandatory.

Also, check if you can take creams and liquids (or any medication) on your flight. Crema solare, ed repellente per insetti possono essere difficile da trovare.

finalmente, buon viaggio!