Wonderful city, almost certainly my favourite. If I could get a decent job there I would.
As some of the others have said, the main square is a rip-off.
I've stayed there a few times and prefer the Vysehrad area in the south, but still only 30 mins walk into the centre. It's mainly residential, with some cool little bars and restaurants populated by locals. Food and drink are dirt cheap (except in the centre). Get yourself across the river to the Strahov stadium at the top of the hill. It's the biggest stadium in the world and is now used by Sparta Prague as a training ground. Well worth seeing. The castle's nice, for the touristy thing. Head out to the east and visit the TV tower with the babies climbing up it. Proper unreal! If you want to check out a game, go to Zizkov and watch Viktoria...excellent stadium! Charles Bridge is great to walk across, and you should walk along the river bank for something romantic. And don't worry about going into any bars or restaurants that aren't Irish (they're shite and expensive anyway), as most of them have menus in broken English. Was there once and found a tiny little bar in Vysehrad, a real dive. Asked if we could watch the City game and he turned it on for us. All these Czechs started joining us and buying us drinks!
Prague is fantastic! Enjoy it, take loads of photos and show your missus one of the most beautiful cities in Europe!
Oh, and watch out for pickpockets in the evenings. I was with some lads there last year and this bird came up to me and started grabbing my cock. Me, being half-cut, wondered what she was up to, until it dawned on me and I grabbed her hand. The woman had my wallet in her hand! Got it back though, fortunately.