If you do a search you will find lots of inspiration as this question comes up regularly.
Invest in a food flask and you can take soups, stews, casserole, chilli, curry etc.
You can have a multitude of variations on the SW quiche. Pasta, rice or cous cous salads. You could get a plain baked potato from the canteen and add your own toppings. Filled rolls, sandwiches or rice cakes with a variety of toppings. Take some boiling water in a flask and you can have mugshots or a hot cous cous salad.
