I like to stay connected while traveling and I found that the best solution is to rent a small wireless router that I can connect my iPhone, iPad and Laptop to. It is cheap, high-speed and easy. Data roaming in Japan is still crazy expensive.
I prefer to use Japan Wireless and order online a week or so before.
You can choose to pick-up the wireless router at the post office in the airport or you can make the ship it to your hotel. When you go back just put everything into the envelope that they provided and drop if in a mail box or at the front desk at your hotel–easy and simple!
I recommend the the 75Mbps from Emobile – for most people the 10 GB limit will not be a problem and I prefer the speed compared to the 3G version.
E.g. 7 days rental incl. shipping (75Mbps from Emobile) is 310 dkk.