These little general stores carry very few things of nutritional value. Avoid buying chips/crisps or little manufactured sweets. As tempting and convenient as they are, they are made by replacing nutrients with nasty little additives and unwanted processed sugar. As a bonus you will save tons of money avoiding the convenience store snacks as they often cost the same as a local meal.

Protrip:

Substitute fresh fruit and vegetables from the market. Most places around the world sell cheap local fruit. If you are looking for some good portable vegetables try cucumbers and carrots.