You will LOVE IT! As far as seasick? It really depends on you, what your doing, what the seas are like, where the boat is, where you are on the boat, etc.. The boats are VERY big and have stabilizers that help with normal seas.
If you have ever gotten car sick, you might get seasick. I would er on the side of caution and get some Bonine or ginger pills. The Bonine works but is also a great sleeping pill for some, so be careful and start on it early to get used to it.
Or if you go to the doctor for anything or know someone that can write you a script without paying for the DR visit, get Transderm Scōp behind the ear patches. They work wonders and cost I think about $40 for your trip. My wife uses them and loves them. I am too sensitive to the medication and it makes me dizzy and dry mouthed.
They are made from Scopolamine, a very powerful alkaloid derived from natural sources. I have to tell you that the natural sources though are from one of the most toxic plant families in the world, the nightshades. So you are not terrified, I would also let you know that other plants in this family are eggplant, tomato, and potato. I will also add that the dose is extremely small, millions of people have used them, and they are considered generally safe. Just read the warnings and side effects. A common one is temporary blurred vision/blindness if your eyes come in contact with the medicine.
As you can see, you have several options, all with some type of mild side effects. If it was anyone other than me...I'd go for the Scop patches AND the Bonine. If the scope patches were too strong, use the Bonine, but expect to be a little drowsy. I don't buy into the homeopathic hype of Ginger, but many swear by it.
As for planning activities, you get a better price to do it in person. BUT, since you are first time cruiser, pay for the protection of booking from Carnival. You can meet new people in your group and have a BLAST. OR don't book anything and just meet people and do what they are doing, whether it is shoping, siteseeing, partying, layin out on the deck or beach. BE YOURSELF AND HAVE A WONDERFUL TIME. Look me and my family up if you like too!