Palace of Justice is the court room, and yes only open when trials not taking place. Take the train and the Bayern pass and 2nd class around 39 euro for two each way or less (I don't have my phone right now to check on exact). Also staying in Nuremberg after or before cruise is a thought, or Munich. We stay at a hotel right by the train station in Nuremberg and very easy to get anywhere in the old town by foot, and plenty of other transportation to get where you want to go. Same as in Munich.