They have offered onboard booking incentives in the past. I haven't noticed it the last few years on my cruises. It was available only on certain ships, and may still be. The incentive was to put down a $100 deposit and receive an onboard credit on the future cruise (if I recall correctly).
I did this on the Destiny in 1997, and also received a bottle of wine on the spot that had Carnival and the Destiny etched into the bottle. I still have it.
They also allowed you to assign the booking to your Travel Agent. I couldn't find any current information. I will try to get some Info about it, if they still offer it.