These days many supermarkets sell 'ready to eat meals'*, salads and food that can be heated with just the simple kitchens that are available in almost all hostels.
*These are often not cheap filling unhealthy options but very good meals, of restaurant quality.
(Just a pan of water kept just under the boiling point with a sealed packet of food will be fine, and you can do that with just a kettle by refreshing the hot water.)
Also you can do a lot with simple fresh food, little of which needs cooking, as long as you know how to do basic cooking.
Cold cuts of meat, fresh vegetables, many kinds of bread and bread substitutes like wraps and ready made toast, ready to eat rice, pot noodles (often called differently in different countries) and much more.
Also look for lunch deals when out in the day, at (the end of) lunch service, many places have very good deals on one hot meal, often cheaper than sandwiches would be, eat then and a simple sandwich at night or make a salad from some cucumber and yoghurt with some grains or pasta.
With a good selection from a supermarket, but also from small traditional shops you can either prepare lunches that can be eaten cold, or you can take the makings and eat the food as is when ready to eat.
Salads if you have a container that closes properly, vegetables that will stay fresh during the activties of the day and some grains you can add cold, or fresh bread and whatever the shops have to add. (I am a meat eater and will often get a meat for lunch rather than cheese.)
City sightseeing is often not good for home-made lunches, as you have to carry it along, have nothing to cool it before eating, nothing to heat it up with when ready to eat.
When in a city, buy your food when ready to eat, but buy creatively, not just the first ready made sandwich you find. And try out local produce, not just of the country but also of its immigrants who often run small lunch shops or even carts and vans.
When you are in a hostel with others, offer to share a meal, help with the cooking and learn from what they do.
If most hostels you stay do not have a kitchen, consider to spend a little more on the hostels and get one with a good kitchen.
Those often come with the basics for meals as others leave them behind, pasta and rice.
And breakfast grains in many shapes and ways.
That will give you a good base to balance some weaker lunches.