Tuna sandwiches can be great and are a good health choice too. Keep everything for this sandwich in your cooler. If the aisle also has some Infalyte, pick up a bottle or two just in case diarrhea happens. If you don't have access to distilled water, boil your own. This is a very user friendly and packing friendly container type.
Heat kills germs.
If you're in a foreign country, be especially careful drink sodas from the bottle or can, and wipe the lip surface with a wet wipe a good thing to have with you no matter how far from home you go. Instant oatmeal and other single serving packets are easy to pack, lightweight, and safe when made with hot water. Also, NCI has NuBars in coffee crunch and vanilla caramel flavors. Pre packaged individually, these meal augmentations can help with fatigue prevention and weight loss protection. For this trekking type of vacation trip, insect repellant is an important provision.