Good packed lunch recipes?

I have no idea what to make for lunch to go, growing up I had a sandwich every day at school. I don't want to buy as much fast food as I usually do.

Anyone have anything good? I already have a go-to breakfast, and dinner is fine because I can figure something out on the spot based on what looked good in the store that week.

Lunch is hard because I have to plan it, and I don't want to pack too little and something that isn't filling. Ideally want something that can be eaten cold.