RCl or CCL cruises are probably the best bet here. Where you go is up to you, but if you live near a port city, a cruise leaving from it may be the best choice as without airfare, it will be easiest on the pocket books of all involved. I would go with at least a 4 day cruise but recommend a 7 since the first and last day really don't count in that they are spent embarking and disembarking.. have fun..
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013