If they are already teens, make that a summer goal to begin learning to cook before they leave the house. Start easy with prepackaged cups of Mac and cheese or ramen. From there move to other simple microwave dishes. Think Quesadillas or boiling pasta to eat either hot or to make pasta salads.
Keep salad fixings on hand and they can make their own salad. Make larger portions of dinner so that there are leftovers. Lots of fresh fruits and veggies in the house, maybe with hummus or dips of choice.
My tween really likes Mexican layered dip. We do fat free beans, skim cheese, and skip the sour cream so it’s actually a decent meal. Or a taco salad using beans or pre-cooked chicken for protein.
You can never go wrong at any age with a pb&j with a side of fruit.