I put an empty Kleenex box next to my daughter's booster seat in my car to use as her own little trash can. Now when she has a used tissue, empty juice box or granola-bar wrapper, she just puts it in the box. When it's full, I can just throw the whole box away.
Now the back seat of my car is neat and free of detritus, and she likes having her own trash can. Perfect!
So simple and smart — makes me wonder why I put up with a messy back seat for so many years. A version of this hack came in a couple years ago as a disposable bedside wastebasket for a kid at home with the sniffles.
More: Road trip hacks