I did that once and my suggestion is to have your friends get off the ship, take a taxi to Muni, fly over & spend the day doing whatever you have planned then back to get on the ship or not! That way they can spend the day doing what they want with their time and not what the cruise ship has planned and cheaper!
When I did it I had already experienced Belize, did not want to be on the cruise & wished I had taken my luggage off & just stayed in Belize & flown home from there.