I wouldn't dismiss Carnival as a choice. We took their Eastern Med cruise on the Freedom and like said before, the Europe cruises take on a more somber tone than the Caribbean cruises (same is true for Alaskan cruises)
So I would venture to say booking a Carnival cruise would actually end up giving you the atmosphere you are looking for. Pick the cruise line that goes where you want instead of worrying about what the atmosphere will be on the ship. Our cruise was 11 ports in 12 days. You are in port all day and are exhausted by the time you get back on the ship.
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
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013