Vegetarian/vegan hotdogs are great for this, since they can last fine without refrigeration for a decent amount of time. When I was a kid my parents would always pack them when we went camping, take a packet out of the freezer on the way out, pop in one of those soft cooler bags, by lunchtime they’re thawed and good to go, and if you have any left, they’ll still be good for dinner! No cooking should be needed (always double check the packet though), just some condiments, or even a tin of baked beans to make a meal of it.