I couldn't find this info before we left but have some to share in case anybody is interested. We had in the past bought a couple of those $100 Future Cruise vouchers which, when purchased at face value and used later, would give you a bonus matching $100 onboard credit on 7 day sailings.
In essence, doubling your money.
We used them, one at a time, on our last two cruises and they were awesome. Come to find out they don't sell them anymore ... there's a 'but'.
IF you book your next cruise during a cruise, they will make you the same offer!
We didn't want to do another 7-day cruise on the Magic (see my Slow service/smelly glass thread), so we booked a 5-day on the Triumph and got a $50 onboard credit per stateroom. Didn't need to pay for the entire cruise, just drop a deposit.
Hope that info helps.