There are many activities and games played during a cruise, all are free EXCEPT bingo and the casino (which I guess should be obvious)
I don't recommend Chefy's game of "ocean tag" but he gives you a good summary of where your extra costs lie. but he forgot one, pictures. They take your picture alot, which they will glady let you buy for a price.
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