The not-so-secret trick here is to take into consideration what she will enjoy more than what you think will make her feel special. If she is not a people person, don’t plan a big surprise birthday party. It will only overwhelm her. Go to the rooftop, just the two of you with some home-baked cupcakes and a few candles. Ask her to look at the stars and make a wish. She will hold on to this moment forever.