Depends on what kind of activities you like. If you just want a nice beach to lay on and have a few drinks, then Valley Church beach (also known as "The Nest" beach for the bar on it called The Nest) I have pictures of it if you would like.
There were plenty of local tour guys to negotiate with. If you just want a beach break, grab a local taxi guy (guys closer to the port entrance), if you want a island tour, go past the taxi guys and talk with a tour guide.
There are 365 beaches on the island. We saw three of them and they were all spectacular. Devil's bridge is an interesting site, but I'm not sure it would be my only stop... Penny also gave you some good options.
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