Hmm...
Try not to be predictable, or anything a girl would probably have tons of or would expect (this includes your stuffed bears, etc.). I'd imagine what it would be like if I recieved the gift. "Ohhh a bear...thanks?"
If it doesnt take considerable thought, its less special.
I'd go with some roses or a bracelet.
Or if you think that's coming on too strong then I'm sure you know her general interests, just something having to do with a hobby or activity she enjoys. It might sound silly, but I got this girl a pokemon plush doll because thats what she told me she liked! Well hey, it worked.
The gift doesn't have to be big, expensive, or anything just sincere. Have ingenuity!