Get her a necklace with the first letter of her first name as a pendant.
1) You don't have to worry about size. Most necklaces are one-size fits all.
2) You can be sure she'll think you've thought about it because it's got her initial on it.
3) She'll think your thoughtful. I mean, a necklace seems like the kind of thing a thoughtful person would buy.
4) You can be sure she'll wear it.
5) I can't think of another reason, but 4's a bad number.