That's why I bring a stack with me as well, and usually keep at least a couple in my backpack (they fold up nicely).
One great thing I use them for is my resort mug - I often will bring it full with me to a park, and when I'm done I just slip it in.
Also handy for the way back to the room - instead of going to the room, getting the ice bucket, and going back to the ice, I just grab one of the gallon zip-locs out of my backpack and fill it up on my way to the room.
They really are the handiest things!
N.E.D.