If sheer calories are an issue, peanut butter (they make it pre-packaged in little cups, or you can pack PBJs) & trail mix or nuts require no refrigeration and are easy to overconsume on the go. Fruit, dried fruit, cliffbars, powdered carbs (like blended oats in your shake) are all portable carb ideas.
If refrigeration is available, sandwiches/wraps as mentioned, pre-peeled hardboiled eggs, as mentioned all good options. Blended chicken and rice shakes if you’re hardcore but these suck cold. If no refrigeration, tuna pouches, shakes, jerky can help fill gaps, but there’s never been a field job I was on where I wasn’t allowed to pack a cooler/lunchbox with icepack.
Sometimes you just have to stand and scarf a cold meal here and there.