Upvote:5
7-11 and Starbucks also have free Wi-Fi.
(I can't recall if McDonald's has Wi-Fi, but they tend to in most countries)
There is also this "Free Wi-Fi Passport" offered by Softbank: https://www.softbank.jp/en/mobile/special/freewifi/en/
According to the site you only need to provide "mobile phone number, password, gender and age group". For that to work, you need to be able to receive the code via SMS with your foreign mobile number, while you're in Japan and connected to their hotspot. You can/need to renew this setup every two weeks.
Upvote:6
I'll work on this answer or I'll change it to an answer wiki if anybody prefers. Please let me know. This is just a rough version so far...
FamilyMart : 3 X 20 minutes per day via email registration. May have seats and power! Look for イートイン "eat in". Can be confusing to get working and staff can't help.
Lawson Station : Easiest konbini Wi-Fi to use by far!
7-Eleven : Most confusing konbini Wi-Fi to use by far. Sometimes Facebook might work even if you think you can't get the login to work.
Aeon supermarkets : up to 15 hours per day? Possibly not "my basket" minisupermarkets.
Michi no Ekis (道の駅), the roadside rest areas which are not on expressways may or may not have free Wi-Fi.