When the weather is good and you can find either a park or cozy bench, you may go to the Department store. Find the food floor.
Usually, there are big department stores around big terminal stations. The basement floor is usually where the ordinary Japanese buy daily groceries, food, bread, confectionaries. You may find SUSHI, TEMPURA, YAKINIKU (Grilled meat), MAKUNOUCHI (Japanese) BENTOU (Lunch box). When you pay, make sure to get HASHI (disposable chopsticks).
I don't like those lunch boxes of convenience stores much. Instead, I buy some of OBENTO or OSOZAI (cooked dishes) from Department stores when I am busy. Cost around from 800 yen depends on what you choose. Since they are not for travelers, you might be difficult to communicate for ingredients or questions. So only in the case you don't have any food allergy, it is recommended.
SHUMAI - Japanese assorted, meat, egg, fish, vege
