MiraCool Cooling Bandanas
A lot of companies sell products similar to these. I happen to own the MiraCool brand.
These bandanas are filled with some sort of water absorbing gel. When you soak the flat bandanas in water, they swell up until they are almost cylindrical. When they’re full of liquid, you just tie it around your neck and the evaporation makes the ambient temperature feel several degrees cooler.
It really works. I wouldn’t walk a round without one.