This weekend are the European Heritage Days, which means that in a lot of european cities, some monuments that are normally closed to the public will be open to visits, usually even for free. Check the EHD's official website.
Two years ago, I took this opportunity to go to see the Vladni Salonek, or Gouvenment Lounge, in Prague's main train station. It has been used in the beginning of the 20th century by President Masaryk, but it's totally closed nowadays. The whole station is undergoing a well-deserved renovation, but this place is in perfect state, completely out of time... You get the chance to see it by yourself tomorrow and on Sunday.
I'm putting up more photos of the train station and this particular space on my flickr page.