Kombu, the leathery seaweed that is used to make dashi stock, is packed full of umami. A traditional way to prepare it is as shiokombu (salty kombu) or kombu no tsukudani. Tsukudani is a method of cooking something with soy sauce, sake and/or mirin, and sugar until it's very dark, quite salty and sweet too. It's a preserving method, since the salt and sugar greatly increase the keeping qualities of the food.

Kombu no tsukudani can be tucked into the corner of a bento box to add a little variety. It's also a good onigiri filling. Properly made and stored in the refrigerator, it keeps almost forever.

Salted salmon, called shiozake or shiojake （塩鮭）, is so ubiquitous in Japan that when people just talk about "salmon" (sake or shake) they are usually referring to the salted kind rather than the raw kind (which is specifically called namazake（生鮭）). Salted salmon is a staple ingredient of bento, used as an onigiri rice ball filling, flaked on top of or mixed into rice, or just grilled.

Salted salmon is cheap and easily available in Japan, but not so outside of Japan. So I've been making it myself for some time now, and it's quite easy. All you need is a typical refrigerator that has low humidity. (If yours doesn't have excess condensation in it, and old leafy vegetables get dessicated in the corner of your vegetable bin, then it's ideal.)