Carrying Extra Clothing
I like to sea kayak in the summer, and have a collection of dry-bags that are just the right size to fit into a kayak hatch. These are now being used to protect clothing inside my paniers. I always do my commutes and trips anticipating a flat or some other situation that will have me off the bike exposed to the elements.
Not a bad idea, if you have the space, to carry some fleece, wool hat, liner gloves, dry socks just in case you have to make a pit stop and withstand the elements.