Evian water in a small aerosol spray.
It may be a few bob in Boots but it lasts ages and the fine mist is always very cool. Added bonus, it doesn't spill in you bag or pocket.
Like Manuka honey, YOU'RE WORTH IT
As well as the little fan it's very beneficial. I think they do something to the nerve endings in your face to alter your breathing (sorry cant be more technical).