A very popular dish in Japan are rice balls, also known as onigiri or omusubi. The onigiri are eaten as a small snack between meals. You can you find onigiri in grocery stores as well as, for example, in so-called Onigiri-Ya focusing only on the production of handmade rice balls.
Onigiri or Omusubi is no kind of sushi. Rice was formed into rice balls to have an easy-to-eat snack on the way. Sushi, however, arose from a way to keep fish fresh by covering it with rice. Rice balls are made with plain white, often lightly salted, rice, unlike sushi which are made from rice that is acidified with sugar and vinegar.
There are written records since the 11th century, for example in a diary that talks about people, who eat rice balls (then called Tojiki) on a picnic. Also, you can find records from the 17th century that talk about samurai that carry with them rice balls, wrapped in bamboo leaves, in times of war.
As in the beginning of the 18th century nori leaves became available for ordinary people, it became increasingly popular to wrap the rice balls in seaweed sheets.
As a popular take-away dish onigiri are made in the many different variations and forms.