You should narrow down your choices by things you know she likes. Also, then you can narrow them down by price (if you guys have a set limit...)
Is she a collector? Does she have a hobby or obsession? I'd try those first.
Is she really into clothes, makeup, and jewelry or is she one who loves an experience - like movies, skydiving, or dance lessons (just to name a few examples)? Does she have a favorite restaurant?
Just some starter questions. Try to give us some additional info about her and we can give more practical advice.