There were some on our cruise who were able to be put on a "waitlist" once on board for the Sunmore excursion with Anna and Elsa, and they found out the morning of that they could go. So it is possible. Go to Port Adventures first thing when boarding and ask.
In Europe last year on my 12 night Med in many places they said they added buses and there was more availability on favorite tours, but I do not think thats the norm, its if they can do it.
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