All depends on what facilities you have, if you eat at a desk, have a restroom, a fridge, microwave and of course what you like
In this house hold, DH will take leftovers as hes working on site, does have a microwave and needs something filling. Left overs will be anything from a full on roast, curries, Chinese ( home cooked ) Bolognese , stew, pie etc. In the summer months I will air fry chicken drumsticks and serve with a chopped salad, quinoa or couscous , make up pasta salads , make quiches or frittata
Myself I usually take overnight oats, or a couple of eggs to scramble and wholemeal pitta. Or I stuff a pitta with salad and cold meats. Sometimes its hummus and veg sticks , sometimes a pasta salad, other times a fruit salad
For me its about avoiding the sandwich, cos the sandwich means crisps and a bar, and ensuring I take something so I dont run across the road and get a sausage roll :)
In other work settings where we have had more facilities Id leave cheese and butter in the fridge, and maybe take a potato or a couple of slices of bread in and have cheese on toast of a jacket spud ( microwaved ) and cheese or even a small tin of beans in