The konbini and the vending machine, use them like a local
If a guest told me they were on a budget, I did not send them to cheap restaurants. I sent them to the konbini, and they came back converts.
The konbini, 7-Eleven, Lawson, FamilyMart, is not a sad gas-station shop, it is national infrastructure. Fresh onigiri rice balls for about 150 to 250 yen, proper egg sandwiches, hot fried chicken at the counter, drinkable coffee, and a microwave and hot water right there. Breakfast for two people for under 1,000 yen, eaten in the morning sun outside, is one of the best cheap meals in Tokyo.
They also solve problems that have nothing to do with food: the 7-Eleven ATM takes foreign cards when bank ATMs refuse them. The multi-copy machine prints tickets and documents. You can ship a suitcase to your next hotel from the counter, takkyubin, and walk to the train hands-free, ask the staff for the yamato form. Umbrellas, phone chargers, socks, painkillers, all there at 3am.
The vending machines, a few million of them, are their own small culture. Hot and cold drinks live in the same machine, red label hot, blue label cold. They all take IC cards now, tap and grab. In summer the cold Pocari Sweat machine is your friend, in winter a hot can of corn soup at a shrine gate is a memory guests brought up in thank-you emails years later.
One etiquette note that keeps you invisible: Japan does not really eat while walking. Buy it, stand by the machine, enjoy it, move on. There is usually a recycling slot right there for the bottle.
Which konbini has the best food?
Locals argue about this forever. 7-Eleven for onigiri and coffee, FamilyMart for famichiki fried chicken, Lawson for egg sandwiches and desserts. The honest answer is they are all good and you should eat from all three.
Can I withdraw cash with a foreign card in Japan?
Yes, the ATMs inside 7-Eleven take basically every foreign card, English menus, fair rates, open around the clock. This is the single most useful cash fact in Japan.
Are vending machine drinks in Japan hot or cold?
Both, from the same machine. A red price label means the bottle comes out hot, blue means cold. A hot milk tea from a machine on a cold night is a small Tokyo rite of passage.
Is it rude to eat while walking in Japan?
Mildly, yes. Buy from the konbini or machine, then stand to the side or find a bench and finish it there. You will notice locals doing exactly that next to the machine.