I think life is too short to keep the frozen salmon in the fridge! :-) So I made a teriyaki salmon bento, which was so fast to cook, that I had too much idle time in the morning... Just be careful not to overcook the teriyaki sauce, because it may have a burnt taste. Also, optional lotus root slices will keep the fish tender and juicy for the time when you can finally devour every last bit of it ;-)
Tuna paste is ridiculously easy to prepare and makes a great no cooking bento, cutting the preparation time to less than 20 minutes, including rice cooking time! Now, that's what I would call cooking on the go! You can also use the tuna paste for a great tasting onigiri or sushi.
Bento is not only rice, but also other ingredients, like... pasta. This recipe came to life when I had some uncooked beef left-overs and just wondered what to do with them. Cooking them as a part of bento was a great idea!
You can buy thin beaf slices in some grocery shops or butchers. They may be called minute steaks or quick steaks and are a great ingredient that you can use to make a quick donburi (a rice bowl with meat and vegetable toppings). It's so fast to prepare, that you can do everything in the time it takes to cook the rice!