Sun Hat + Warm Hat
Sun Hat: This can be anything from a simple baseball cap to a round brimmed beach hat, as long as it's light and will scrunch down easily for packing. A neck cover like the one shown is also useful.
Warm Hat: Any ski hat will do, try to get something that covers your ears. A windproof lining such as Gore-Tex Windstopper is also useful, and don't go too large as it needs to be able to fit under a helmet (most normal ski hats will).
Quantity: 1 sun hat, 1 warm hat.