You could arrange to stay in the Pulitzer hotel - top class 5* overlooking the Prinsengracht - you could also organise a boat trip for the 2 of ye, wither privately or on an organised tour.
Lovely restauarant to go to is Chez Georges (I think its on Herenstraat) - very cosy restaurant with really nice food (we had the 7 course surprise the last time we were there) relativley expensive, but still cheaper than any equivalent here in Ireland
Sometimes there's ice-skating on rembrandsplein at this time of year - bit packed to be really romantic I think though.
Personally speaking, just walking along some of the canals and seeing the meeting of 14 canals at Reguliersgracht & Herengracht - its all lit up and night and looks very romantic, **sometimes its not about money!!
Disclaimer - **sometimes it is!