A slightly different question... What about prices on board? I know pre-paying is generally cheaper for things that can be purchased before the cruise sailing, but are there any tips for things that can only be purchased on board? For example, spa treatments are usually cheaper on port days. Duty-Free alcohol prices generally do not change, so you might as well buy early if you know what you want. (BTW, if you find it cheaper in port, snap a picture and you will get a price adjustment.)
I'm not familiar with RCL, so fire away. I'm particularly interested if there's a discount on laundry offered. NCL used to have a deal where you can fill a bag for one price. Trying to stuff as much clothing in that bag as possible was fun.