I just grabbed a couple of the salad boxes, and the dressing containers (+ lids) from my local grocery store, filled them up, and put a little personalized label on top.
You could put anything inside, but here are a few ideas from my kids favorites to get you started:
almond butter sandwich
durable fruit (like an apple or banana
Don't forget to include a couple of napkins and a baggie for the garbage! I usually include a little produce bag so they can throw the trash into it when they are done.
I now do this on all our flights and road trips, varying the ingredients inside by how long the boxes need to stay intact and what their favorite snacks are at the moment.
The other reason these are great is that instead of having to stop repeatedly on road trips because the kids are getting hungry, the kids can snack on what they want, as they are hungry. It has saved me from many "I'm hungry right now!!!" breakdowns from my youngest, who never alerts me to when he's feeling hungry until we're past the point of no return.
Happy summer travels!