If you are not beach and water people, then my suggestion would be to ignore the ship excursions. Go to the port/terminal entrance and barter with a cab operator to get a tour of the Island. You should be able to get someone to drive you around and show you the sights fairly cheaply. (less than the ships tours.) Just make sure you don't pay them (or fully pay them) until they return you to the ship.
I can't really recommend anything in particular since we always do the snorkeling thing (Trunk Bay) I'm sure there are some plantation tours or something that might be interesting. Elsewise just have a taxi take you to the downtown shopping district.
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, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013