If you decide to tip upfront (and many people do) you might find a small service that you wish to ask performed and while handing them your tip say "Do you think you could have cookies in our room each evening" or whatever you want....
If you don't have anything special you are asking for, you can always just hand them the money and say "We're glad to meet you and are really looking forward to a great week."
Carnival Elation March 11
Carnival Imagination Sept 07
Carniival Sensation Dec 06
RCI Sovereign of the Seas Sept 06
Carnival Miracle Sept 05
Carnival Glory Sept 04
Carnival Fantasy Jan 04