You don't mention what ship, but for a 7 night cruise, I think a soda package is around $42, but I'm not sure, as I haven't cruised Carnival. You would need to drink at least 3 sodas per day to have that pay off.
You don't mention your age, but in the casino, I think you have to be at least 18, if not 21. I would check your daily planner for the tournaments in the casino. Be advised, international law prohibits the casino from being open while you are in port.
You will not be allowed INTO YOUR STATEROOM until around 1:00 pm. You will be able to board prior to that and go to the public areas, but your room probably won't be ready until at least 1:00 pm. You don't mention which port you're leaving out of, but it can depend how many ships are in port, and what you mean by easy. My last cruise, we left out of Port Canaveral and the line was really long to check in on that ship on another cruise line.
2007 - Majesty of the Seas
2009 - Enchantment of the Seas
2010 - Freedom of the Seas