My husband got me these cooling towel things (made by the same company as the ones Susan showed upthread). They might not hold up for a very long time outside when it's 100 degrees, but they help me out a great deal in the house.
The ones I have are 13" wide by 37" long so I can wrap one around my neck twice. You just get it really wet, with cold water, then wring it out and it says to 'snap' it, then just wrap it around your neck - or wherever. It stays cool for quite a while for me and I generate a LOT of heat.
I am no longer able to tolerate being out in the heat, so I haven't tested them out outside. Just a few minutes in heat that's not even 80 and I fade pretty quickly and get sick, due to an auto-immune challenge.
I'm really getting a lot of mileage out of these! He got them at Lowe's. I would be interested in wrist-size ones, though. I wonder where I can find those. I need all the help I can get with the heat. I hate summer so much!