There are some great ideas in here that I'm taking to add variety to my kids' lunches!
Our kids' preschool has a really strict food policy -- no sugar, no salt, no nuts and no disposable packaging. Practically this means no cookies, candy, muffins, dried fruits, crackers, chips, nuts or nut butter of any kind, etc. It also means no prepackaged single serving cheese sticks, yogurt, applesauce, fruit pouches, veggie sticks & dips, etc. unless we unwrap them or transfer them to another non disposable container. While annoying at first, this forced us to come up with a frugal, healthy, easy lunch / snack packing system that we've continued even though our older child has moved on to elementary school which has no restrictions.
So, here's what we do...
Dinner leftovers are our primary lunch strategy -- this covers roughly 50% of lunches. When our kids fail to eat dinner or eat only a small portion of what they've served themselves, the food left on their plates gets loaded directly into the lunchbox for the next day. We tend to cook a little more than we need to make it more likely that we'll have extra for lunches.
When we don't have dinner leftovers, we pack a fruit + veggie + protein + starch.
Fruit: Whatever is cheap or in season. Banana, apple, grapes, oranges, berries, melon, etc.
Veggie: My kids are pickier about veggies & prefer them raw so we just cut a bunch over the weekend and pack whatever we have. Carrots, celery, lettuce, cucumber, etc.
Protein + Starch: Often we can cover this with dinner leftovers -- e.g. chicken and rice, bean burritos or quesadillas, pasta, etc. If that's not possible, we have easy to pack proteins ready to go and pack them with a tortilla or slice of bread as the starch. Hard boiled eggs, lunch meat sandwich, cheese (cut off a block), beans, tofu, nuts or nut butter sandwich (for the elementary school kid only), etc.
The only extra food prep / cooking this requires is precutting some veggies & hard boiling a bunch of eggs at the beginning of the week. I have also occasionally cooked a big batch of beans and brown rice & made a dozen burritos for the freezer which I can pull out and thaw anytime for those days when we've got nothing. This system is mostly healthy, cheap, easy, and meets the strict preschool requirements. It also strongly discourages picky eating -- refused dinner foods are almost always lunch the next day.
For our own sanity, we use lunchbots bento boxes ($$$) instead of our regular tupperware to pack lunch because it means only 1 container & 1 lid per child to wash every day rather than 3-4 tupperware containers each. Plus, our older kid only gets 20 min to eat, and we've found that he tends to eat a wider variety of food when he only has to open 1 lid to get to all of it versus only eating 1 of the packed items when he has to fumble with a different lid for each food item.