If she's a fun gal with a sense of humor, she'd probably appreciate a gag gift. Not something insulting, of course, but pokes fun at whatever situation she is in, or comically makes fun of gift giving. A bag of socks is a fairly common one in the States. I've seen things like "My brother went to New York's finest gift shop and all he got was this lousy shirt" before. Just get something that can remind her of you and your qualities. Women like that kind of thing. It can even be cheap enough to buy a 'real' gift as well... or better yet, a companion gag gift.
Of course, if she's not one for laughs, I'd say stick with the polo shirt.