I take the leftovers of whatever I had the night before in a flask (soup, stew. casserole etc), or some home-made spicy hummus with baby veg (carrots, baby corn, snap peas, tomatoes), or a pasta salad in a box or something. And I always take several pieces of fruit and sometimes some Quorn slices or something else free. And plenty of stuff to drink (water/soda/diet Coke/coffee or whatever). Anything to avoid the canteen crappe.