Omlette eats well cold, I tend to have mine about half veg, onion, bell pepper, mushroom, kale, leek, sweet potatoe works well if you are going to be active/want more carbs. Easy to eat without cutlery without too much mess.
Other than that shove stuff in a box and add a fork, I had; coleslaw, hardboiled egg, cold roast chicken, liver pate, salad leaves, bell pepper and celery today in my packed lunch.
You know all those pictures of Adam and Eve where they have belly button? Think about it..................... take as long as you need........................