Japan is a funny mix of wonderfully advanced technology, and oddly outmoded machines. It's been some time since I've seen both fax machines and dot matrix printers in action.
While you might expect the WiFi to be free and flowing, this isn't really the case. And staying connected while you're in Japan is really important- at a minimum, to get around and make yourself understood, you'll need Google Maps or Hyperdia, Google Translate, and if you're as bad at understanding the exchange rate as I am, something like XE.com.
So, plan ahead and make sure you've got yourself sorted. You can choose to roam, get a SIM, or rent a WiFi unit. We've found renting a WiFi unit the best and cheapest option to keep two phones, one iPad and one laptop connected.
Renting a WiFi sounds slightly bananas when you first consider it, but it's really common and very simple. Your WiFi rental company will package up and send your unit either to an appointed collection spot at your arrival airport, or directly to the place you're staying. Chances are you'll need WiFi to get yourself to where you're going, so unless you're arriving later in the evening, an airport pick up is the best bet.
You'll see a few options when it comes to selecting a unit- I've always gone for the cheapest. I also use many more GB than the 'allowance', after which the speed is meant to be throttled, but have never noticed a difference.
Your WiFi will probably come with a case, a charger and cord, some instructions, and a return envelope, already addressed and all set to go. It should be charged and ready to use. All you have to do is to find the WiFi network on your phone or laptop and enter the unit's password. This is also a good time to make sure that data roaming isn't on, assuming you're not going to be roaming while you use the unit.
While you're using it, it's a good idea to carry a spare battery. In areas where the signal isn't so strong, the battery can run down a lot more quickly. Plus it means you'll get home without issue if you stay out for longer than you'd planned.
When you're leaving, you just pop everything into the return envelope and mail it from a postbox at the airport. Make sure that the envelope is as flat as possible- I have had to do a few last minute re-packs in order to get the envelope into the slot. But really, it's a cost-effective and handy solution.
You'll find a number of companies offering this service, and the prices should be pretty competitive. I've used Rental Wifi three times, and had a good experience- plus good customer service when I couldn't get a signal on Yakushima Island.