banana smoothie won't go grey if you add a few drops of lemon juice or add a bit of apple juice.
Wraps are always good sandwich alternatives.
I wouldn't do rice salad personally, but hoe about pasta salad, couscous salad, potato salad, cooked cold chicken drumsticks, a tuna, olive and cherry tomato salad with pasta
filled pita breads
falafel
mini quiches and tarts, mini cornish pasties (home-made)
Toasted cheese sandwiches (they are quite nice cold)
cold slices of pizza
bean salads
hot stuff in thermos food flasks.
Spanish omelette (tortilla)
then add things like fruit, yoghurts, cheese cubes, little containers of dried fruit, crackers, bread sticks, crisps (sometimes), smoothies, grapes, and any other snacks your DC like.