Hey Jorez... Welcome to Cruise chat!
This question shows up a lot here. There is not too much you HAVE to spend extra on. Except for gratuities, most of the typical things on a Carnival Cruise are included in the fare. Extra things are - Photos (these are expensive with a 5x7 costing $7?? and a 8x10 more like $12. Needless to say they take your picture every time you move... but you don't have to buy any of them. Casino costs (obviously) Spa treatments, bar bill (including soda, but Tea,coffee lemonade are all included) the Steakhouse on ships that have them is an extra cost, $30 each, but you don't have to eat there as all other food venues (except the coffee bar which also serves cakes and chocolate covered strawberries) So you have a huge variety of food to choose from for free...The movies on the pool deck on ships with the seaside theaters and in your stateroom are included (although there are some PPV channels) are included.
So you COULD sail without spending anymore than your tips as extra... but... you will spend money on drinks and you probably will end up spending $ on a couple of pictures.. but its impossible to tell you what you will spend as everyone is different.
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