In some respects I’m a typical Japanese person, since to me there’s nothing as soul-satisfying as a hot bowl of miso soup. At home we have miso soup at least a couple of times a week (see my week of miso soup  series for some ideas.) Miso soup with a bento lunch is great too, especially at this time of year when you feel a bit chilly inside even if the roo is heated.
There are many kinds of convenient instant miso soup packs out there. I like to make my own ‘instant’ miso soup balls though. They are dead easy to make. All you need to do is combine about 1 to 2 teaspoonsful (for an average size miso soup bowl) with whatever ingredients you have on hand. All you need is a source of boiling water at lunchtime, which most offices have. Put the miso ball and ingredients in the bowl (or you can use a mug), add hot water, and let it sit for a few minutes while the ingredients expand and flavors amalgamate. This technique is often recommended in Japanese bento books with a healthy or macrobiotic focus, since instant miso soup mixes are often loaded with preservatives and MSG and so on.
You can buy miso with dashi stock already added to it, but I usually just use a regular white miso. You can add a pinch of dashi stock granules to the miso if you need a bit more saltiness and umami.
There are more exotic (to most people) dried Japanese ingredients that could be used, but I hope this list gets you started. A miso ball soup is only limited by your imagination!