Here's 3 examples in order of effort (use Granny Smith imo):
Peanut Butter + Apples is a combo that already is known to work (use Granny Smith), adding some toast to the mix only improves it.
Goat Cheese + Apples + optional Arugala is a very solid combo, with the Goat Cheese pairing almost perfectly with apples, and again, bread makes this a meal more than a snack. Shoutout to megabanette for bringing it up already.
Goat Cheese + Apple + Salami / Prosciuitto + Arugala + Brown Mustard in a nice whole wheat Sourdough is imo a perfect sandwich.
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Apples add a nice element of crunch, tartness, and sweetness depending on variety that is otherwise missing from sandwiches, and really improve the texture overall if you put them in the right place. Apple + Cheddar also works.