The cruise lines have many ways of separating you from your money. It's just a matter of how much of it you want to give them. As Mike and Katlady have said, please remember your tips they are an important part of the crew's income. Everything you spend on board will go on your Sign & Sail account which can be put on a credit card, so you don't need cash for that. (I think the casino may be the exception to the rule.) Depending on your itinerary and comfort level you might be booking shore excursions through the ship. Check these out on Carnival.com. You may want to book them on-line pre-cruise. They will show up on the S&S account as an expense and a credit.
Other things you can spend money on:
Photos, art auction, golf simulator, internet, souvenirs, gold by the inch all sorts of things.
Your cash expenses will be mostly on shore; excursions, shopping, dining.
The maitre d' is not included in your S&S tips. They will remind you of this and you can choose whether and how much to tip him/her on the last night.
Enjoy your cruise.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Alaska 2014 - haven't picked a cruise yet
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012
Celebrity: Summit 2011
Princess: Ruby 2010, Caribbean 2013