Anything that is served with a dip is a huge hit. Ranch dressing, tartar sauce, honey mustard dressing, whatever. If they can dip it, mine would always eat it. Chicken nuggets, fish strips, mozzarella sticks, carrot sticks, celery sticks, string cheese, cheddar sticks, bread sticks.
Breakfast food at any time but breakfast is always a hit. Sometimes I will make scrambled eggs and serve it with sour cream and salsa and thats how you know its lunch instead of breakfast.
They used to seriously love mini-pizzas made on english muffin halves with a little bit of spagetti sauce and a slice of american cheese then slid under the boiler for a few minutes.
Sometimes I would make my own "Lunchables" and make cheese and crackers and lunchmeat cut into little pieces.
Other stand bys: Toasted cheese sandwiches, mac and cheese, ramen noodles with a handful of frozen veggies from the freezer thrown in the boiling water with the noodles, chicken noodle or tomato soup, bologna and cheese sandwiches with the mustard made into a smiley face, Rice a Roni in chicken flavor with a little leftover chicken chopped up and thrown in.