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.