Don't get her a humidifier. Trust me, girlfriends don't appreciate that.
Depending on how long you've known her, I think jewelry would be a safe bet (I say depending on how long you've known her because if you've known her a while, you'd obviously want to get something a little nicer). I don't know any girl who doesn't like jewelry. Or, if there's something she's mentioned that she'd like to have, get it--nothing's better than letting a girl know you listen when she talks!
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