I wanted something that could be easily eaten on the field trip, held in one hand and not fall apart or have a lot of little pieces to deal with. Burritos are perfect for that.  You could practically just take it in your pocket.  It's easy to make ahead, and personally I think they taste just as good cold as they do hot, especially if they have just simple ingredients (I don't think I'd like a cold steak burrito, but bean and cheese does just fine).  I just buy organic canned refried beans, and I shred big blocks of cheese.  I'll try to do a video one of these days about the rolling, but it's really quite easy - the trick is to not over stuff the burrito. Just a few spoonfuls of beans, a sprinkle of cheese.  I microwave these enough to get the cheese melting into the beans and the tortilla nice and soft.  Then fold the sides in, and roll.