Good morning. I hope you are having a great time in Japan. The TravelPass will work if you are not planning on using the phone too much. Once you reach .5GB, you will be slowed down to up to 2GBs for the remainder of the 24-hour session. However, it will still work for calls and texts. And then if you have WiFi available, that would definitely be the better option.
Does this information help? -Vanessa