Best idea (for me) was to ask my girlfriend what are the things she likes, if you ask your girl now, she'll know your gonna get her that, it must be a surprise that you can get her something that she wanted and not having to ask you for it.
Or you can be as perverted as me and kenny and have her go to your house while your in your birthday suit :3
But srsly, you can also be her slave for the day, treat her like a queen, do stuff for her, etc.
If she's having a party, go to her party and be with her the entire time, show some luv boy!!