Carrying a mobile hotspot can really enhance your trip to Japan.  If you arrive at Narita airport, go to the Telecom Square booth on the way to the train station.  At this booth, rent a wifi Mobile hotspot device.  A 7-day pack is 7,350 yen, or you can pay 1,260 yen per day.  The device provides wifi access with WEP or WPA security for up to 5 devices.

The device should work everywhere except in the underground subways, including on the bullet trains.  With google maps on your smart phone, you can navigate around town without carrying tour guide books and without worrying about your cell phone roaming costs. This wifi rate is much cheaper than paying for internet access for multiple devices in a hotel or cafe, and of course it's mobile.

The battery charge only lasts about 4-6 hours if you leave it on continuously.  So you might want to only turn it on when you are actively using it.  It boots quickly.  Great for those 1 hour train rides.  The charger is very compact.

When you arrange your rental, make sure you get the expiration date and time correct.   You don't want to end up with the device locked out before your trip is over!  And note, there are other providers in the airport besides Telecom Square.

Travelers to Haneda Airport. The provider detailed below services both airports and the odd location throughout metropolitan Tokyo.

This company ranks high in customer service and quality equipment. They have a location in the airport itself and show that location on a map you can review at their website. The website is in English.  They have a free delivery service and a premium delivery service (choose if flying into Haneda or if your flight arrives very early or late when the kiosk is normally closed.) The rates to rent the WiFi seemed very competitive to the other vendors.