My theory for putting in a good bento box and to be nutritional it is. I like to make a ratio of two type of vegetable, which can be pickle vegetable, steamed, stir-fry, and always combine them with some fruits as the last bite of the bento box.
Also, one part of the starch. In this case we can use ramen noodles, soba, any type of leftover that is starch. The great idea about bento boxes is that we always have something warm to eat and something cold. So, always remember to that the starch and the proteins have to be together in the same compartment at this point to be reheated. Then we have the second compartment, which in this case, fermented kit we put some fresh fruit and sushi and some fruit that we can enjoy cold.