If it’s a hot summer and comfort is more important to you than style you may be interested in this Solar-Powered Cooling Pith Helmet.
Not only is the Pith Helmet (AKA a hat) a good sun shield it also incorporates a fan to keep your head cool. Fan is operated by solar power in the day and batteries at night. I suppose if you wear one at this you’ll be cool in one form :).
Available from Hammacher.
Update: The Solar Powered Coolith Pith Helmet is no longer available at Hammacher.