Depending on your budget... for high end and truly memorable. Book a suite at the Peninsula for one night - a harbor facing one. Arrange the view, propose and arrange to have a nice private dinner served en suite.
Other end of the spectrum... I don't think you can find a better view than the Peak. Take a picnic with some of her favorite foods. Spend a little bit of money on top shelf chocolates from Maison du Chocolat.