When I holiday in Japan I always rent a pocket wifi, partially because Japan doesn't generally have free wifi everywhere like some other cities (that's changing a bit lately though), mostly because my wife and I are social media whxres who can't survive 2 hours without internet.

http://www.globaladvancedcomm.com/pocketwifi.html

Can be a bit pricey on a student budget. On the bright side they ship it to the post office at narita airport so you can pick it up upon landing.