If you find yourself penniless in Tokyo, you can survive on eating
1. cheap sushi from the 7-11 (still better than anything you can get in Waterloo, trust me) < 4$
2.
gyudon (literally "beef bowl") in fast food chains. < 5$
There's probably other cheap things to eat (but it's certainly not the melons!), but gyudon restaurants have the advantage of pictures to point at. Gyudon is delicious! It's thin slices of beef marinated in a sweet sauce with onions of rice. in Even I, the queen of I hate weird Asian meats, loved it. I wake up in the middle of the night craving it, that's how delicious it is.