What a sweet person you are! And welcome to our forum! Depnding on where you are and how much money you can spend, here are some suggestions:
1) Guerlain samples arranged in a heart-shaped pretty basket, box, crystal bowl, so that she can choose the one she likes best and, thus, eliminate the problem of choosing when it comes to a bigger bottle for her birthday, Christmas, etc;
2) A trip to sniff the mall -- a bonding experience that will give you a better idea of her likes/ dislikes -- just pop into each store and smell their fragrances away (please take with you a handful od coffee beans to chew on in-between the sniffings, but some stores will offer them free!);
3) A bit less expensive? Bath and body products, like shower gel and body lotion, but the whole thing of Aqua Allegorias is not that bad pricewise, there are a lot more expensive or "niche" fragrances out there and your Nordstroms or Von Mauer should have them in stock!
4) Lines to pay attention to:
Dolce & Gabbana Light Blue,
Christian Dior Dior Cherie,
Lolita Lempicka L,
or, search "vanilla" in the Directory.
5) Something might just "appeal" to you, like my DH gave me a bottle of Moschino Couture! for Valentine's last year, and I like it just because he went through the trouble of choosing it himself.