Here's a list of Men's Hats for Spring/Summer 2012 on Amazon (though it seems a few women's hats snuck in).

Perhaps you could get away with a Newsboy-style cap? I know I've seen people wearing them, as well as hats similar to the ones in the link, but I'd have to say from memory most of them were younger "hipster types."

Baseball caps are definitely not as common as here, but you could perhaps get away with it if you purchased one with a German football team's logo on it.