An insulated sleeve placed inside a backpack is great for leftovers. We have two about the size of a 10 by 13 envelope. We have a large baggie in each and put our leftovers in and take them with us. (We like to save room to split a dessert.) We usually eat one meal a day and snack or eat leftovers the other meal. We bought the sleeves on
Amazon as well as the very flat frozen blue ice things that go inside. You can carry water bottles in these too. (I read reviews and bought the highest rated sleeves. I just looked and they have no brand name on them in case you were interested. I know they cost $14.00. They work great.)