Do you have access to a fridge or a microwave or a toaster oven????
You can bring anything you want - leftovers, soups, stews, casseroles, salads, fruits, anything goes.
If you don't have access to any of those - it then changes. foods that need to be refrigerated need to be refrigerated - you don't want food poisoning. You can't take anything that needs to be warmed up.
A good thermos will keep foods hot or cold for you if you need one.