If you have a good insulated lunch bag, thermos, small Tupperware containers and ice packs you can pack almost anything.
The suggestions you’ve received below are great.
My kids like leftovers so we use the thermos a lot. They have taken chicken stir fry, pasta with meat sauce and veggies, soup, homemade mac & cheese, BBQ chicken, mashed potatoes, etc. Warm quesadilla or tacos wrapped in foil, BBQ chicken wraps with lettuce & cheese or turkey wraps made with spinach lettuce, cream cheese and craisins.
For snacks they like fruit, veggies, hummus w/ pita bread or crackers, yogurt, trail mix, cliff bar, breakfast bars, cheese, pretzels, hard boiled eggs, tortilla chips w/ guacamole and dried fruit.
Today I packed them leftover dinner from last night. Pasta with alfredo sauce & parmesan, yellow squash, sweet bread & cantaloupe in a separate small container. For snack I gave them Odwalla bars.