Recommendation at a convenience store
I personally would recommend the obentos. The obentos sold at convenience stores such as 7 Eleven, Lawson, Family mart have very nutritious selection of bentos. You will always find at least one dish of vegetables, meat and rice. They are all very fresh as well. You could get a bento everyday and you would not get tired of it since they have a wide variety. It is also possible to warm it up so very convenient.