Lockable boxes that enable you to charge your mobile phone when out. 20 minutes 100 yen - lock it in - walk around the shop - come back - unlock - a decent amount of charge in it. How convenient is that?
They have them in electronics stores mostly, also at Narita airport - very convenient when arriving back with a flat battery.
It's not new, nor is it unique to Japan, at BKK airport you can charge you phone (free) while you wait for your luggage to arrive on the conveyor belt.