If your friend love cooking, you may buy some herbs such as thyme, rosemary.... or nuts such as pine nut....
If she is your close friend, you may buy "victory secret" underpant for her
Centrum Vitiamin is cheaper than in hong kong
If she is not too close friend, you may just get t-shirt, refrigirator mangnet or a small sounviner with san francisco logo at China town or Fisherman Wharf.