pens... so they don't leak on the plane
paper... for my kids to play with while waiting, notepads, postcards, flashcards
food... cheerios, fruit, pretzels, etc.
diapers and wipes in my backpack/diaper bag... can easily grab the ziplock with a few diapers and wipes and don't have to drag the entire bag into the tiny bathroom with me (especially when flying... can also then talk daddy into taking kid into bathroom while I watch bags)
snack sized ones... medicine bag full of Motrin, sudafed, aspirin if needed
contact lens case... put my contact lens case in there so it doesn't get dirty or get lens solution all over everything
an easy travel size of wipes without the expensive cost of travel sized wipes
in emergencies, also have used them for wet suits, dirty clothes/sox (!!!) going home, clothes from accidents and sick kids....