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.