You can keep cool by adding a few drops of peppermint oil to water and shaken well in a spray bottle. The peppermint's contact to your skin cools you down instantly and it lasts quite a while too. Make sure you test before spraying yourself down, probably around 5 drops may be enough, although it's all down to your own preference. Just don't use on children, as it may be hard to tell what their threshold for the coldness of the oil is.
You can also take a bath with a few drops of peppermint oil to cool you off. Use common sense and stay out of the sun after using because the coolness of the oil may be deceiving!