Like Dwayne said.. nothing they automatically add to your account. (Again you can modify this amount if you choose to do so). So you CAN in theory get on the ship and not spend anything additional for your trip. However, this is rarely done (at least I have never known anyone to spend $0 extra).
Extra items are - Photos... its like going to Disney World.. they take your picture every time you turn around.. this can rack up big bucks if you try to buy them all- DON'T... if you choose to buys pictures pick and choose carefully. They encourage you to buy them throughout the week as they are taken. My suggestion is to wait until the end when you can look at them ALL and choose a couple you feel you must have. We normally buy the formal night family picture (when we are all dressed up) as this is like going to have a family picture taken.
Soda and Drinks- coffee/tea, milk, lemonade, juices and hot chocolate are free sodas and adult drinks are an extra charge
Spa visits- anything in the spa is an extra charge. This can get expensive so be careful is you are on a budget.
Steakhouse- $30 pp extra.. but worth the price for at least one trip during the cruise (in my opinion)
Casino money (of course)
Excursions- you can buy excursions online before the cruise if you want to know an amount before you start but you can also add them on the ship.
Bingo. They play at least one round of bingo each day. count on $20 a go...
I'm sure I am forgetting something but those are the major items. They normally make it clear when something is an extra fee but when in doubt, ask. Also be careful in the spa, several treatment have a base price and anything they "offer" you can be an up charge. Make sure you know what the extra fees are before agreeing to add anything to a treatment.
So if you are not going to drink soda or alcohol and don't do any excursions or want any pictures... it is possible not to incur extra charges.. but like I said, I don't know of anyone to pull this off.
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, Celebrity Silhouette 2013