That's a good question. Once you have your sail card, you should be able to come and go as you would in any port of call. Unless they restrict that due to embarkation proceedings. Maybe they have a different entrance you can use?? I'd ask before I got on if I were you. Unless someone who has done this can answer for sure. It would be a tough call between getting on the ship and relaxing or holding off and touring San Jaun...I think I would elect for the ship though...
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