4- A Bandana. This has multiple uses, (if you have a red one - wave for emergencies) use as a wash cloth, sweat band on the head, shade over the neck if you tuck it under your hat, wet it and wrap it around your neck to cool off, a handkerchief, a water filter, first aid bandage, etc., etc.
Of course carrying these items means that you should have at least a
daypack or a fannypack with you. Going out with nothing can lead to