I always toss a couple plastic zipper bags in my suitcase and carry-on.

They come in handy for everything -- from wet bathing suits after a hotel swim, to a dirty diaper on the airplane. They also keep souvenirs dry and unscratched, and come in handy for muddy shoes or liquid gifts (like maple syrup or olive oil) that might otherwise ruin clothing in a suitcase if the glass breaks.

Don't be sad if you can't find a zippered bag: any plastic bag will work. In fact, larger bags are great for containing muddy shoes, or can be used as an impromptu "day bag" in a pinch.