Goose - Not knowing which ship you'll be on, I'll give you general answers.
Some Carnival ships have the "supper club". Basically, it's a special dining room with a $30 pp charge. I've never been interested in checking this out, as the food in the formal dining rooms is always wonderful...so why pay?
Some ships have little "coffee/dessert" bars, and you'll pay there. You'll know it's a $ because there will be signs posted.
Otherwise - all food is included in the cost of your cruise. This includes:
Formal dining rooms - all three meals
Lido Deck buffet area - all three buffet meals + extra desserts put out during the day. Some days, they have "Make Your Own Sundae" areas.
Pizzeria - Great pizzas, calzones, Caesar salads - Open 24 hrs. a day.
Soft-serve ice cream & frozen yogurt - Available 24 hrs. a day.
Deli - Really good Reubens and other type sandwiches.
Room service - No hot foods, but a great assortment of sandwiches, salads, Danish, desserts, etc.
Sushi Bar - If you like that sort of thing.
All beverages on the ship are included (except alcoholic and soft drinks and the specialty coffee bar.
Coffee, decaf, iced tea, hot water & tea bags, variety of juices, lemonade, fruitpunch, water. All of these are from self-serve machines.
Coffee, decaf, iced tea, hot tea, water, milk, skim milk, chocolate milk, hot chocolate, juices. (Also - espresso and cappuccino at no charge.)
Most of the above listed beverages at no charge - 24 hrs. a day.
On sea days, around 3 or 3:30, they also have "Tea Time" held in one of the small lounges. Usually a piano player or small combo playing. There'll be a variety of teas, coffee, petit fours, tiny sandwiches, mini cookies, etc. served by white-gloved attendants. You don't have to be 'dressed up' to attend. I go in my shorts. A nice respite from the activiites on the ship. No charge.
WHEW! I think that's all.