Fruit and salad sounds a bit meager, unless they have beans too, or you can grab sides like rice, fries, bread, pasta etc.? I'd consider bringing a few nuts, seeds (pumpkin, hemp, sunflower, sesame), toasted soy beans, or sprouts to sprinkle on the salad if they don't provide anything along those lines. I wind up eating something from the salad bar at work most of the time, since the vegetarian dish almost always contains cheese (argh!) and I always come home starving in spite of there usually being chickpeas, lentils, sweet corn, beans, the occasional rice or pasta salad and some kind of starch.
If you have the means to cook your own food and don't necessarily *have* to eat in the dining hall, the thermos might be a good idea. I may look into that when I go back to uni. Pulses are cheap and make soup that doesn't leave you hungry afterwards. That includes peanuts.