I recently spent 3 nights at the Mamaison Riverside Hotel. I travelled with a group of friends some of which also stayed at the hotel.
Communication before arrival was a nice touch and they acquiesced to many of my requests. The building both inside and out is beautiful, the room was very well finished with tasteful decor and a particularly fabulous bathroom. I really liked the in room safe with a built in plug socket, very clever.
Lovely views of the city.
It's a decent walk to Charles Bridge but you can get straight over the river toward Praha 1 fairly quickly and the walk gives you a chance to look at the fantastic architecture and scenic views.
I would definitely stay here again and although I went with friends it'd suit perfectly a romantic get away.
Oh I must mention the bed, it was one of the most comfortable beds I've ever slept in.