My Go-To gift for girls is Hair Pretties
Especially for girls that I don't know so well or know their size. I like to make a few different types - barrettes, pony tail holders and headbands. I also like to include a little gift card to Target or Borders so they can pick up something they like too. It usually goes over well.
I like to try and make at least one or two Hair Pretties that corresponds to the next holiday. So as you can see below there are 2 Valentines Day ribbon ponies:
(OK, not the best photo, taken at night with my camera phone.)