Towels everywhere!! Towels in the diaper bag, towels in the trunk, towels in the back seat. I never leave home without a big, light colored towel.
You never know when you might need one. They are useful for doubling as a blanket; for cleaning up messes; for a changing pad; for drying off after running through the rain to get from the store to the car; to cover car seats to keep the buckles from getting to hot; to cover the stroller to keep out light, rain, heat, etc; for privacy if you breastfeed; for putting baby down on the floor; and so many more uses (be sure to add your ideas in comments)
Being prepared for a variety of emergencies with a big, light colored towel-- it works for me.