On those ships with ice rinks, is it just for the shows, or can you actually go and skate? I didn't pay any attention to it when I was on Freedom last year, but I have a new cruising buddy going with me on Navigator next month, and she seemed really excited about it. Navigator's page says "challenge yourself to the ice-skating rink", so I'm thinking so, but would it be something we need to book ahead of time, or just show up?