Carnival has a VIFP club (Very Important Fun Person), to reward repeat passengers. You can join the program before you've even cruised for the first time, and every day you cruise is a point you gain, which will unlock different benefits.
After your first cruise you'll be offered members only discounts, an e-newsletter, and a complimentary drink at Sunday brunch. As your points add up you'll be eligible for priority check-in, carnival gifts, arcade credits, laundry, and cabin upgrades to name a few.