Jewellery is always nice. Also, cuddly toys, maybe one of two of her most wanted CDs. You could do a lot worse than have a quiet word with her mum/sister/best friend.
You could make a grand gesture - take her out for a winter picnic, or to a show, or something else she'll enjoy.
DO NOT buy her underwear (it won't fit, plus she'll either think it's too tarty or too dowdy), perfume (unless you're absolutely certain of her favourite) or anything 'domestic', eg. a DVD player, TV, set of saucepans etc etc. You also take a risk buying clothes generally, for the same reason as underwear).