Also note that not all CCL ships have spa cabins. The conquest class ships have the Thermo pool but may not charge for its use. The Dream class and the Splendor DO have spa cabins and their spas have more features that allow them to charge a "day spa" fee if you are not staying in the spa cabins.
However on all the ships the fitness center equipment and the steam/sauna rooms in the locker room area are free to use.
This is one of those areas that Carnival is starting to "nickel and dime" you to death (of course its more like $20 and $50 you to death... Spa treatments and now some of the spa facilities are extra charges now.
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, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013