During any kind of outdoor activity, such as hiking or picnicking, I always keep a bandana or two on me. They make great cooling tools (pour some water over yours and wrap it around your neck), impromptu serviettes, protective headwear, and purveyors of dry picnic goods.
If your life is more exciting than mine, they can also be used as:
~ face masks during sand storms
~ bathing attire (ladies, you might need, say, three)
Any other ideas?