if you live with each other, make her breakfast or dinner and then tell her she is beautiful all day long, see how many times you can make her smile, give her a massage! girls want their man on their birthday. good luck!!!
Don't know whether your girl girl love stunning items, if she do, then bought her some jewelry, a bracelet, necklace, pendant etc.made with crystal might be good choices, if not, bring her some items that is practical to release her annoyance, a cute scrapbook might be good for her! :)
Well,i think you are the best gift for her :) but girls usually like beautiful dress and jewelry . maybe you can have a look at this fashion site http://www.mixmoss.com which is my favorite one .hope this helps .