My mom and I went on a 3-night cruise the weekend before Halloween back on 2009 (when they were celebrating Halloween on all October cruises). One of the days was designated for trick or treating...the hours were posted in the Navigator. Around the ship, they had various trick or treat stations set-up with CM's manning the candy (the CM's were dressed up as well). In the evening there were costume contests (both for kids and adults). I'm sure there were a few other things as well, although I would have to dig out the Navigators from that cruise to see exactly what other events were happening.
We had a good time, and there were several folks dressed up. I'm sure the cruise that actually fell on Halloween saw even more costumes!