if you want to buy her a cake.. i can suggest na mo buy ka sa cakes r' us at sm. nice ayo ila cakes. personalized
flowers will work as well....
if you're thinking about giving her gifts (material stuffs) .. you yourself is the best person to know about this kind of stuff kay he's your gf. so mas naka ila ka niya so you know what she likes/dislikes... or ask her bestfriend/friends/siblings..
^_^ good luck bro...