We go camping a lot and my kids love building their own sandwiches. sometimes I'll use a cookie cutter (metal) to cut the bread in shapes and then I just pack into containers different fixin's. Like, turkey, tomatoes, cheese, ham. Or Those individual fruit packs/jello packs are a great serving size. Veggie sticks with a dip. Cheese sticks. the pre-cooked chicken nuggets (my kids aren't always a fan of them cold, but sometimes). We've also done the mac & Cheese things. Barilla has small packets of different flavor that you mix & cook, you could cook ahead...my kids sometimes will ask for our leftovers cold. Spaghetti O's....i'm dating my self to college days. I remember just eatinig those straight from the can. Raman noodles (cooked ahead). you can mix in just about anything you want.
I also just took a taco salad to a family renion. Mixed upon serving. I cooked the meat the night before. then carried it in separate containers until serving. Lettuce, cheese, Kidney beans, meat, western dressing, & chips.