My kids love wraps for lunch. Sometimes it is good old fashion PBJ, lose the bread and use a tortilla wrap. Other times they have turkey, lettuce, ranch, cheese in a wrap or black beans and cheese. I've even done cream cheese and strawberries in a wrap. They hold up well and you don't have to worry about that crust that for some reason is an aversion to most kids.
A great way to serve fruit like watermelon, for example, is to use a cookie cutter to make it into stars, high heels, princess crowns, etc. Big hit!
Another favorite is s'mores sandwiches. Two graham crackers made into a sandwich filled with nutella and marshmallow whip. Or some times, I skip the whip and just throw in a handful of mini-marshmallows.