How do I surprise my girlfriend on her birthday?

My girlfriend is turning 23 next week. Her family is throwing a big party for her and invited some of her closest friends. They plan on surprising her and of course, I'm part of the act. I haven't bought anything for her yet, since I'm also busy planning for my own birthday surprise for her. I have read a lot of suggestions in a foreign dating blog but nothing that was stated there really stood out for me. I want it to be a special and memorable day for her. How can I do this? Any ideas? Thanks!