On some Carnival ships (I think 5) you can buy Future cruise credits.
You give Carnival $100 and when you book a cruise (could be on the current cruise), Carnival will credit that $100 back to the balance due (doesn't count for the initial deposit), and you will get an On board credit of up to $200, depending on length of cruise you book.
$50 OBC for 3-5 day cruises
$100 for 7-8 days
$200 for 12 days of more
The credit is per cabin and singles only get half (Booooo!).
You have 5 years to use the certificate and isn't supposed to be used on cruises already booked.