Quesadillas are essentially the Mexican version of grilled cheese sandwiches. Because my children like anything with cheese, they scarf these little goodies down no matter what the filling is. (evil laughter... ha ha ha).
Seriously, the following are some sample fillings--my kids even know that broccoli sometimes occurs and they still eat them. I lay one tortilla down in the skillet, sprinkle some shredded cheese on it--lightly, spread on/sprinkle on another filling if desired, and top with more shredded cheese. Lay another tortilla on top, grill as for grilled cheese, and cut it three ways for my three kids. Perfect lunch.
Mixed cheeses, including cream cheese
Beans--slightly mashed pintos and black beans work great
Finely chopped cooked broccoli and/or cauliflower (leftovers of course)
Finely chopped cooked chicken or beef (leftovers of course)
Mild salsas (spread very thinly or it will get too soggy)