There's not a lot of black hat activity, but there's plenty of good- to neutral-natured messing about. The "Wall of Sheep" is a great example; if you send something that looks like a username/password on the public wifi, someone will put it up on a big display (though they'll obscure some of the password field) along with the other 'sheep'. Not "black hat" but definitely not the kind of thing you want on a real account.
As for burner clothes, the only thing I've ever heard is to not wear company-branded clothes - wearing an obvious Google t-shirt is a great way to attract attention you may not want.
Speaking of which, I've long wondered how hard it would be to make the wall of sheep display ASCII art or something by letting it sniff bogus credentials....
