The NHS has this useful section on 'Lunchbox ideas and recipes' - https://www.nhs.uk/healthier-families/recipes/healthier-lunchboxes/. At the bottom of the page there are lunchbox tips as well as some recipe ideas. Ideas include (click on the recipes below):

Cheesy coleslaw with wholemeal pitta recipe

Tuna mayonnaise and sweetcorn sandwich recipe

Hummus and salad wrap recipe