I freeze my waterbottle, and use an insulated bag (got it at an outlet for $5). The frozen bottle (or two!) keeps the sandwich, and another beverage nice and cold in the lunchbox, about 6 hours (what I do each day), and then can also be consumed later as well a few sips at a time as it thaws (can be refilled and refrozen if wastefulness bothers you). You can also drop in some instant coffee, tea, or flavor drops later to it.
From my pantry, I have "emergency" meals for hurricane warnings. Marinated bean, rice or grain salads are just fine warm if they are vinegar or citrus based, and kept just a little cool. Get a basil plant, to kick it up a notch. Squeeze some lime or lemon over some black eyed peas, or cannelini beans, think celery, chopped onion, bell pepper, hot peppers, touch of olive oil if you want to add a fat, which is nice. Buy Suddenly Salad in the box, and add a cubed mozzarella, chopped tomato, chopped cucumber, jarred olives. Or take it mexican with cilantro. Near East Taboulleh salad is also nice, just do your add-ins.
I see no issue with a banana, peeling an orange, enjoying other whole fruit or washed grapes consumed warm. Wrap sandwiches if kept cool can be cream cheese, plus veggies, slice of smoked turkey, anything, and is very portable no worries of smushing.
Most days my lunchbox is a granola bar, whole fruit, 3-4 drinks, and either a microwaveable soup or a sandwich. Even cheese and crackers are fun. Making a sandwich the night before like egg salad, chicken salad, or my favorite pimento cheese gets it nice and cold already for another hour or two of life in the lunchbox.