The only extras you need to consider are tips, and they must be earned by the staff. Things like tap water, OJ, Iced tea and lemonade are always free. So is the fat free yogurt by the pool. Bottled water and soda are not free.
You can eat pretty much whenever you want if you are OK with the buffet or room service. You will have a set seating time in the dining room for dinners, but other than that you're free to eat as much as you want whenever you want. Room service is free, but it is courteous to tip a few bucks.
Some people pack sodas in their luggage and then they stick them in their minibar to save a few bucks. It's not permitted, but very few rules are enforced on RCI. I'm not advising you to break the rules, mind you.
Where people end up spending a lot of money is in alcohol. If you avoid the $9 drink of the day specials you can be well-fed, never thirsty and never pay an extra dime all day.