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.