say. wake up early morning and send a bundle of flowers to her home first;
put it at her home, then leave with her to a special place where she never went to., you need to find it out before and checked. properly it may be a beach.
Write some thing about love with her on a pink paper and put into a bottle (old strategy but useful), then store it under sand..
then go to lunch., properly a Italian restaurant with less people. Book a heart shape steak for her. If there can arrange you to cook in front of her, then execellent. you cook for her no matter it is good or not., she will die hard... and you sing a song or play piano (even a toy piano), just for funs!
then relax with movie, any sweet one and let her to reply on your shoulder.
then go shopping (but in fact you bought and put a gift at a shop), and let the shop keeper to send to her while you are singing a birthday song. hope you can request the shop sales to sing with you. the gift should be not too big...
then have a high tea at Peninsula Hotel (around HK$140/head), book two seats near window to enjoy the view.
tell her more about yourself in childhood.
then go to dine out at anywhere, not very expensive one at Soho in Central.,
with another bunch of flowers there in advance.
Comppleted and say you love her.
BUT
"if you are adults, book a nice hotel and stay overnight!" Enjoy yourself!