Bread with cheese (and almost whatever else you might put in there). Wrap it in foil and it will be ready to eat whenever.
Cold pasta with cheese - or even normal pasta with tomato sauce cold is great. My Italian friends eat this for lunch regularly.
Cold pizza slices are great - I normally make simple pizza using baguettes and tomato sauce (passata) with cheese. One baguette is enough to make pizza for my family of 4.
Pasta salad - pasta cooked and cooked and then salad of your choice - even cold cuts of meat to well in here and it’s economical to make a huge amount and put it in the fridge
Actually thinking about it, I eat a lot of lunches made from cold leftovers and I think they taste brilliant without having to heat them up. Obviously things like soup, or ramen noodles and other hot liquid meals don’t taste good at all cold.