Sakura Mobile Potable Wifi (Pocket Wifi) Rental Service
A rental service provider of data SIM and portable WiFi (Pocket WiFi). Although a bit pricey, their various plans of portable WiFi are available in wide areas as they are based on the network of DoCoMo. Shipping fee will be lower if you pick it up at a store.
The largest network coverage! Usable in various places, even in subways!
docomo's mobile Wi-Fi uses their network, which covers the largest areas in Japan and is stable and consistent. With their network, you can connect with the internet even you are in buses, trains, and subways.
Although they don't offer any unlimited data plans, considering using it mainly for SNS or Google Maps, their offerings are just sufficient for your needs.