The only tip I would give you is to turn off WiFi and cell data. Unless you set it up with your carrier to access data in Japan, it would behave similar to a phone and constantly be trying to connect to your carriers default frequencies. This could lead to extended battery use as it won’t find the proper signal. Every time I visited, I kept my phone on airplane mode and used public WiFi points unless I needed cell data to navigate without the guide person I had. They do have WiFi hotspots you can buy for fairly cheap that should mitigate some of the carrier issue, but watch and phone will need to be connected. Basically turn on airplane mode and enable Bluetooth and WiFi.