To add to F-mattox's suggestion, you can also take the train to Pompeii (Pompeii Scavi station)
Then see Pompeii and return to Naples on the same train that takes you to Pompeii. Give yourself about an hour and a half for the train trip and taxi/walk from the Naples train station to the ship.
Doing both, you can probably spend 3 hours in each place if you check hydrofoil schedules and train schedules so that you do not waste time waiting for the next train.hyrdofoil...
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