I wear "driving caps" a lot, and picked up a wool balmoral for a very good price last year at the Games, but honestly...when it's hot...

I wear a bandana on my head a LOT. I've been working out in the park, heaving heavy stuff around for a couple of months, now and the bandana saves the top of my head from getting fried. If I wore a "real" hat, it'd come off while I'm throwing. If I slathered sunscfreen up there, it'd get in my eyes and burn....fuhgeddit. My solution is a bandana....usually a dark blue one from Orvis.

......but I've thought about the noggin wraps from sportkilt. I still might try one.