I guess it's a matter of personal taste and also of what's your relationship like, but being a girl, I can share the type of things I like to receive and maybe that'll help:
1) something the two of you can experience together, like concert tickets or museum membership or a double spa passes - it's cool cause it's a gift that create a good memory and a good experience for the both of you, rather then just being an object that's just for her.
2) something that shows how well you know her and how you listen to what she say - you've been going out for a while now, do you know what her favorite film is? maybe a DVD of that, or a book by her favorite author or anything else she mentioned that she likes and has to do with her interest and hobbies.
3) something that you made and that is unique to you and can celebrate the connection you have with her, if you play an instrument maybe you can write her a song and sing it, if you are good with craft, build her something, I know someone who made a computer game for his girlfriend's birthday.
Again it vary between different people, but in this situation I would find an impersonal so called romantic gift like flowers or chocolate or even jewelry insulting. The whole idea of a gift to me is not something that would show that you spend a lot of money but to show that you actually now and like the person you are seeing.
Things to avoid:
cloths and shoes and so on - at lest you really know about fashion and know her taste (and size) perfectly. This can be really insulting if you get it wrong.
Sexy clothings and sex toys - again depend on the relationship, but if you never did any of those before, this is really not the right time to introduce a sense of adventure into your sex life. You might want to give her something nice by that, but a lot of women see it as a sign that you are bored or unsatisfied with their body or their sexual preformence.
A pet - again if not discussed first - this is a big obligation and she might not really be into it.
In case of doubt, you can always ask her, I know it ruins the element of surprise, but it's still better then disappointment.