Thank you all for your comments. Actually she is a very kind, altruistic, humble, gentle and sweet girl, and not someone out for shiny, sparkly and expensive-looking things. And finding a new girlfriend is definitely not an option because the sex is great!
I found out that she doesn't like chocolate, so that is out. I also found out that she ordered something from Japan for me. She likes to spoil me with gifts, most of which I don't need (which annoys me because she is wasting money. I told her I am going to get a box to put all the useless things she buys).
Yes, maybe a piece of jewelry with both our names engraved. I forgot what day we met, how can I let her tell me without she noticing I forgot?
The week-end off somewhere is a good idea, but I think for Valentine she wants something she can keep.