Depending on her overall tastes, there are lots of options available:
If she is into theatre, or music in general, there are some great gifts that you could get her:
-- Concert / threatre tickets
-- A CD / DVD set of her favorite musical or band
-- A t-shirt or calendar of a favorite artist
-- an iTunes giftcard (great if she uses an iPod)
-- Good quality headphones (yet again if she uses an MP3 player in general)
There is also more generalized stuff you could get, depending on how well you know her:
-- A new purse / wallet
-- An eye catching tote to go shopping with at the mall
-- Jewelry
-- Perfume (find out what brand she likes)
-- A good quality bath set (who doesn't like soothing aromas and a hot soak?)
-- Chocolate, good quality jelly beans, or any other favored treats.
I'm avoiding bigger things like an iPod Touch, Flip Mino, etc. since (in my family, at least) those are generally birthday gifts. Also, I'd -guess- you would know if you were getting something that costly.
At any rate, I hope this is helpful. Good luck!