the trains are so cheap. or they were when i was there. i would ride around and stay on a loop and pop off to get stuff. i didn't spend hardly any time in tokyo, but convenience store food in japan is the absolute jam.
"Lawson" and "Everyone" were my fave, as far as chains go. the onigiri at Everyone was a little ball if heaven.
post offices have international ATMs if you wanna get/use cash. where i was didn't really take CC, so that was a thing i had to manage. also, look for 99¥ shops for cheap gifts and souvenirs. their cheapo nicknacks are hysterical sometimes. i still have this insane cartoony hamster-shaped cutting board i bought at one 15 years ago. i love looking at it and explaining it origins when guests say, "wtf is that??"