I'm a huge fan of using leftovers as part of lunch packing, especially if it's something I know the girls will be excited to have. Leftovers equals a sandwich alternative. Of course sandwiches are an obvious go to staple lunch item, but options are a plus to keep things fresh.
My kindergartner had a leftover hand made chicken tender from her special dinner out the night before her Nana and Papa went home to New York. I heated the chicken in our toaster oven, chopped it up, and placed it in our thermal container. GL did report the chicken was still warm by the time she had lunch. Everything else was cold, so those items were packed in panda.
This bento came home completely empty:
Chicken tender in a thermal container. Panda Bento had broccoli (also left over), peas, joyful rabbit contained honey mustard, crackers, strawberries, and grapes!