Full UPF 50+ sun protection in a very lightweight package. Fast-wicking, lightweight fabric alongside a soft, fold-able peak let you stash this cap so it's barely noticeable in your pack, but it's there when you need it.
Buff says:
A trekking cap with a long visor to offer an even greater protection from the sun. Comfortable, breathable and stretchable, it is a stylish proposal for sun protection during hikes and mountain treks in good weather. Easy to pack and carry thanks to the ultimate packable technology.
Buff's Pack Trek Cap has an excellent UPF rating of 50+, so whilst weighing less than 30g you get full sun protection and a decent sized peak. The peak is soft which isn't ideal in windier conditions, but it's a trade-off to allow you to completely fold the cap away into a very small package.