Dobry den The Wookiee
Try Cihelna - pronounced see-hell-nar (you'll need to book +420 257535534), it's right on the river, the food is excellent and it's pretty cool. There's also a pool table in the bar upstairs. It's smart but not too expensive and is the sister restaurant of and next door to Kampa Park (+420 257532685) which is one the city's best and most expensive.
All cabbies will know both names so no probs finding them.
Na shledanou
Couple of distinguished restaurants around the castle......................and fantastic pubs/eateries along the castle side of the river, with great views and 17century buildings with all the trimmings. In Prague you can't go far wrong.
I do hope the romance of your weekend isn't ruined by the hordes of British stag and hen weekends which seem to be ruining Prague in pretty much the same way that they turned Dublin into a drunken vomitorium a few years ago.
Prague is a beautiful city, and deserves so much better.
Me and Wifey, told friends, to do what LC has also recommended.
It was a great success apparently, so that's 3 seperate couples telling you to get your ass over the river.