Keep it simple and easy to clean up. Aluminium foil and baking paper are your friends.

Here’s a few I like …
Salmon filets smeared with a teaspoon of green pesto, wrapped air-tight in alu-foil and put in a hot 200C/375F oven until the foil packet expands with steam ~25 minutes. Put some plain asparagus wrapped separately in foil, about 20 minutes. Serve with some boiled potatoes or rice.

Chicken with lemon zest*, salt and pepper; alu-foil as above. Spread a shallow baking sheet with baking paper or alu-foil, spread chopped peppers, onion, carrots, and thin potato slices and drizzle very lightly with olive oil. 20-25 minutes at 200C/375F. Salt after removing from oven (not before). Could substitute potatoes with some instant cous-cous done on stove-top. (*scrape a lemon against a metal grater to get zest). The roasted veggies do better with a convection oven (circulating air). If so, be sure to drop the temp from 200 down to 180 or 170.

Sausages with potato-mash and broccolli—all on stove top; sausages might be better if you have a grill. Easy easy easy. Not as healthy, but tasty, filling, cheap, and local sausages sometimes are very nice.

Stir-fries are easy, too. If your hostel has a wok, then just get some meat, olive oil, fresh veggies (peppers, broccolli, sugar peas, carrots, etc.) a can of koko-milk and a packet of green curry or other seasoning. Serve over rice.

If you’re not very experienced in the kitchen, hostel kitchens probably arent the best place to begin since you don’t have many ingredients to work with and the cookware can be, well, random. So maybe you want to try at home or a friend’s place or something, watch food network, etc to get yourself a little experience.

Pasta seems to be the preferred choice for backpackers; simple, light, easy to carry, just drop it in boiling water for a few mins, strain it, and dump a jar of sauce on top. Rice is another good staple to carry around, though slightly more complicated to cook. You can do plenty to flavor it up and take away the blandness, like saute some diced onions, peppers, garlic (or any veg of your choice, mushrooms are a good one, a small can of tomato is good too) in a little oil then add rice and vegetable stock (or bullion dissolved, or tea, or some kind of flavorful liquid).

Salad with a light dressing is also easy to make, just cut lettuce, onion, tomato and whatever else you want, put salt and pepper, squeeze a fresh lemon, drizzle some olive oil and toss.