They say when you are in love you look at the world through pink glasses! So, why not use this idea literally! Make your life pink at least for an evening!
So, if your want to fulfill this romantic idea, you should:
buy pink sheets, napkins, one-time plates, pink champagne, caramel, pink grapes
cook shrimps or squid
attach pink bows to bed, mirror and some other items of furniture.
For decoration you may choose any room - kitchen, bedroom, even bathroom. But in my vision it should be anyway bedroom, the place where all your problems recede into the background and your mood changes into romantic.
Arrange everything and invite your girlfriend, without betraying your plan even by a single word!
Imagine, how surprised she would be!