Soccer Onigiri Nori
Onigiri, or those triangular shaped balls of rice is pretty ubiquitous in both Korea and Japan. Simple to make, easy to carry, and eat a few of them, and you’re pretty much set for lunch.
You can’t avoid them, especially in Japan, as they are sold in all the convenience stores and most markets. They, of course, come in different flavors, from the all-time classic umeboshi, the ones with salted, sour plum in the middle, to bonito fish flakes, sea weed, etc.

The ubiquitous “conbini onigiri”
Many people just make em at home, as I tried, but I just ended up making a mess everywhere. I might need more practice.
For those parents who want their little kid to have a special onigiri at school, here’s a “soccer-nori” packaging, in which all you have to do is apply carefully these sea laver on to a really round ball of rice.

wish I had some as a kid.