Tom is absolutely right, especially for a 3 dayer. You'll find lots of interesting things to do. They usually have cooking demo's, ice carving, kitchen tours, (you can see how they serve so many great meals so effortlessly), digital camera seminars, and almost anything else that might interest you. I've even taken music lessons aboard ship! The bottom line is that you can do as much, or as little as you wish. If you want to get away from the crowds, and read a book or just watch the sea, then the Prominade Deck is usually the answer.
Don't fret, you'll have a great time, and probably wish it was a 7 day cruise when it comes time to disembark.