One of the first things I do when I arrive in a new country is walk around the local shop, get the lay of the land, and pick up some essentials.
It's usually not feasible to eat out all day every day, and for me there are a few things I look out for:
1. Cheap and convenient sources of protein: tinned tuna, yogurt, sliced meat, etc.
2. Foods that don't require a kitchen or cooking to prepare: oats (also very cheap), bananas, dark chocolate, rice/corncakes, nuts and seeds, etc.
Usually when you eat out, carbohydrates and fat are easy to find on the menu. Enough protein to meet daily requirements... not so much.
That's why I usually base the meals I don't eat out around protein. Corncakes with tuna or powdered peanut butter (with water) is a great example.
Or a topped yogurt bowl. If you can fit protein powder in your bag, it usually mixes well with yogurt, or you can pour a pre-mixed shake into oats and leave it to sit - making protein oats. Throw in a banana, nuts, seeds, dark chocolate, fruits... easy as can be.
I also carry a set of cutlery with me - call me crazy but it comes in handy!
When eating out, first of all... enjoy yourself! Once again, look for dishes with reasonable protein content. If you're looking to limit your calories, fats are where they lie.
Chefs are heavy-handed with oil, butter, and the like because it makes food taste so damn good! Try to limit dishes where sources of fat are key ingredients for a low-calorie option. Think lean protein and carbs.
An easy way to manage your calories abroad is intermittent fasting. This also reduces the frequency of your meals - meaning less eating out or less planning.