Sandwiches are fine.
Use good bread, and a range of fillings as variety - cheese, ham, cream cheese, mashed avocado, tuna, whatever you like.
A bit of cucumber is fine. Or carrot batons, or some fruit, or cherry toms
A chicken drumstick
A yogurt, or sometimes a bit of flapjack.
Things I have included in DS's lunchbox from time to time: a samosa, a sausage roll, a wrap instead of the sandwich, one of those sushi snack packs, little packs of Twiglets, chunks of cheese, chunks of fresh pineapple, little felafels, dolmades, cold fish fingers, slice of cold pizza, mini quiche.
For small children i think it's really important to give them things that they can manage easily, and eat quickly and efficiently, and not overwhelm them with a liunch box full of loads of fiddly ingredients.
Lunchboxes are 5 out of 35 meals a week. Not every one has to be totally balanced, and many children actually like having the same thing every day. Also the school day is long and they may not be eating snacks at break. Don't fill them up on endless calorie-free sticks of celery, carrot and rice cakes which take ages to eat and don't give them energy. The school day is tiring.
Healthy lunch box means no fizz or sweets or bars of choc. Not banning ordinary food.