For those too cheap to buy pocket wifi and staying in the main city's, japan offers free wifi for tourists for two weeks
http://wi2.co.jp/tjw/english.html (android or ios app)
In the beginning you only get access to about 60,000 hotspots but if you visit one of the listed stores (http://wi2.co.jp/tjw/premiumcode.html) to get a "premium code" you get access to 200,000 hotspots. Those flying to japan via JAL can get a premium code from JAL (http://www.ar.jal.com/world/en/freewifi/). HOWEVER, this only applies if you book through JAL directly. If you book through AA you don't get a premium code.
http://wi2.co.jp/tjw/english.html (android or ios app)
In the beginning you only get access to about 60,000 hotspots but if you visit one of the listed stores (http://wi2.co.jp/tjw/premiumcode.html) to get a "premium code" you get access to 200,000 hotspots. Those flying to japan via JAL can get a premium code from JAL (http://www.ar.jal.com/world/en/freewifi/). HOWEVER, this only applies if you book through JAL directly. If you book through AA you don't get a premium code.