I agree with
@JAYSCS that you ought to figure out how offline maps work and start using them. They happen to work better
everywhere, not just when roaming.
Also, checking esimdb.com, you can get an eSIM with 1GB of high speed data and unlimited 128 Kb/s data (good enough for Google searches, finding waypoints in Google Maps, summoning an Uber car, WhatsApp text messages, Apple iMessage text messages, etc.) valid for 7 days in Japan for...about S$0.95. If you want the same deal but with 30 days of validity that'll be
shockingly expensive: about S$1.15.
Enjoy your trip.