You can get prepackaged meals that you can eat after submerging the packet in hot water for a few minutes. Hot water is easily available in most place, even more so if you carry a flask.
Protein bars are filling and quite nice to eat. Dried fruits like apricots or dates are also a nice calorie rich option. Bread and jam/pb are the usual travel staples. Milk powder and cornflakes/cereal is also an option, but not one of my favourites.
You can also carry out forms of bread like pretzels, bagels or focaccia which will last for a few days or even more.
Don’t forget that experiencing local food is part of any travel experience :) you’re not going to travel to Iceland to just eat in the future. So it might be a good idea to explore local supermarkets for some local fruits, bread, snacks and so on. Hopefully some Icelandic person will chime in with more suggestions.