We are on a low income but didn't qualify for FSM until now, due to the specific rules. Things I give DD, in one of those lunchboxes with the separate compartments:
Leftover pizza slices
Macaroni cheese, lots of cheese in so it's really cheesy. Done in a dish, cool & cut into squares/rectangles. Served with tomatoes, carrots, cucumber slices etc in a separate container to keep them fresh.
Big scoop of hummus with veg for dipping.
Peanut butter & jam sandwich (they're allowed peanuts in recipes etc, but not as loose nuts).
She loves brie, so sometimes as a treat, she gets a slice of brie wrapped up, & little tub of crackers. And a few grapes. Makes her feel very fancy!
I quite often boil up half a dozen eggs, cool them, store in the fridge, & put one in DD's lunchbox if it's a little low on protein. Cheap & easy. They usually last about 3-5 days. Good for sandwich fillings as well.
Leftover sausage, or other meats. You could do leftover veggie alternatives. I'm veggie but DD & DH aren't.
Flapjack. Soft recipe, homemade, lots of dried fruits etc. Nuts (if you're allowed). Cheap, filling, reasonably healthy.
I almost always put in a portion of carrot sticks, cucumber & cherry tomatoes.
Sometimes I put in some mixed dried fruit, plain popcorn, a few pieces of chocolate covered raisins (I buy a big bag & put in a few per day). DD likes dried apricots, cherries, cranberries, apple slices, pineapple, mango.
Obviously, fresh fruit as well.
Crisps once a week.
Chocolate biscuit a couple of times a week.
Obviously, watch the balance of protein of fat/protein/carbs.