I love apple slices for a snack or with a sandwich for lunch. I've never packed them and taken them with me because, well we all know, they start browning after a short while. I've only enjoyed them at home, right after slicing them so that I could consume them before oxidation takes over and makes them look unaPEELing (see what I did there?). By the way, this is called enzymatic browning, if you are looking to start some dinnertime conversation.
I've heard of adding lemon or lime juice, I think this is good when adding to a fruit salad. But just for packing in a lunch, I like the saltwater tip: