Good arm covers
I'm not sure I'm 100% sold on the cooling properties of these things yet. But to be fair I haven't worn them in weather where I really was at the edge of overheating. And I assume that those marginal situations are when they would be most effective.
However, regardless if they cool as advertised, for the price they are still nice compression sleeves for warm weather. Protects from UV better/more consistently than sunscreen. Reflects heat from the sun. And they have the sensation of being well ventilated.