As a kid, and still occasionally:
Crunchy Kraft Peanut Butter and 2-3mm sliced Granny Smith apples. These really are good.
My go to now:
Crusty white Vienna with sesame, sliced sandwich.
Ham off the bone
Swiss cheese
Sliced tomato (put the salt and pepper on the tomato, sprinkle some dried basil on tomato if you really want to make it sing) Forgo tomato altogether if not eating immediately.
Sliced pickled onion
Hot English mustard on one side
Piccalilly mustard pickles on the other side.