I often try to plan for a lunch that minimizes microtrash. This generally leads to wraps instead of sandwiches, and stuff like PB&J or cream cheese & jam instead of things that can fall out onto the ground. Elaborate sandwiches with lettuce and tomato seem like they create a lot of microtrash.
That said, for fall and spring trips, I've started doing a stew, chili, soup, or other hot dish heated after breakfast and stored in a ziploc inside a little lunch cooler. If there's room you can put a loaf of bread on top of the stew and it'll provide more insulation and warm the bread. Served in paper hot cups or bowls with bread, torts, etc. this makes for a really hearty lunch if its a chilly day.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse