We were tempted to book another cruise last week while we were on the NCL Dawn. They were offing $100 on board credit for the current cruise if we put a $250 deposit down on a future cruise.
We could book up to four future cruises for a total of $400 in credit to our current bill. The future cruise deposits were fully transferable to friends or family and we had four years to use them.
The only catch was the future cruise had to be 6+ days.