Cotton 'driving cap' from MINI USA Motoring Accessories. Unstructured, one-size fits all, elastic back, embroidered MINI logo.
I leave it in my car as an emergency cap, but I can also stuff it into my jacket pocket (or back pocket, or camera bag pocket). Not much else to say. Not much eye protection from the sun, but good scalp protection from the sunroof and cool air.
Cool air: coming soon to a sub-tropical region near me.