Sunscreen, Sunglasses, plenty of smiles, and half the clothes/shoes you think you're going to need. Not sure what your itinerary is, but you may want to bring a light jacket/wind breaker. Have a wonderful cruise!
The most important thing to bring is a good attitude. I find that if you're expecting a great time, that's exactly what you'll get. Experience everything -- do it all! Don't hold back. You'll probably come back anxious to book another one. I always do!