We did this. We showed up and just walked up to the ticket window and bought the audio tour. You need to leave them your driver's license as a deposit for the audio player. We then just walked around (shared the player between the two of us) and listened. If you want a real person guided tour you can get one, but they are more expensive and you then have to wait for a tour to be available. To be honest you with I can't remember the cost (and this was last June!) I want to say it was 11 Euros? maybe a couple of more for the audio tour...
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007