A ship tour can be arranged through the shore excursions desk while on board. I think on our last cruise the cost was > $100, I don't remember as we did not participate. There are also Galley Tours - watch the Fun Times during your cruise.
We have not sailed at Halloween but have seen many costume pictures from other cruises.
A Bad Day At Sea [with power] Always Beats A Good Day At Work
Coral Princess - tour starts 5/31 in Fairbanks
In 2010 I did the Behind the Fun on Imagination. Was very enjoyable. Working in the hospitality industry myself, I really enjoyed seeing how they make it all come together on a ship. Back then it was $55 for the tour. They only take like 16 people or so and it sells out quick. I know this past December when I was on Imagination again, the friends I was with inquired on like day 2, and it was already gone. So once you are on the ship, make a super fast run to the Excursion desk to try and book it.
Disney Dream - Dec 2012 4n Bahamas
Carnival Imagination - Dec 2011 4n Western
Carnival Imagination - Nov 2010 4n Western
Costa Atlantica - Dec 2009 7n Eastern
My opinions are mine and mine alone. They do not represent the opinions of The Walt Disney Company nor any of its subsidiaries.