Miso soup is a traditional Japanese soup consisting of water with soup stock (Dashi), miso paste, and some other ingredients depending on family's recipes. Great thing about miso soup is that besides soup base (Dashi and miso), you can add and change whatever ingredients you like, giving great variety and options on its taste and nutritional intake. In addition, it's very easy to make! Miso paste is made by fermenting rice and soy beans and is a great source of vitamins, protein, and minerals like calcium, magnesium, manganese, and selenium.
Here is a good article introducing some super foods in Japan!