Traveling with 2 young boys I have learned the helpfulness of having different size disposable bags on hand. Zip lock bags, small trash bags, medium size food bags, and plastic bags with handles. The medium size food bags and zip bags are great for splitting up snacks purchased on the go between young kids. That $10 popcorn you purchased won't get dropped, both kids can eat on the go, and what is not eaten can be saved for the next days snack. Also if you stay at a hotel with a free breakfast and your little one gets full before he eats that muffin he begged for stash it in a zip lock and save it for later. I was headed for the trash anyways.
Small trash bags in the car or plane are a must. If you have kids you know someone is bound to get sick, spill something on themselves or on you, or just make a big mess! With that said I also always keep paper towels in my truck on road trips!
Whether its a car trip, plane trip, or walking around a city they all the different size bags come in handy and store easily!