I really think the app might be the best choice. Any bookings close three days before the cruise and my experience is that in the last couple of days before that there was quite a bit of availability popping up for excursions and some tastings and so. Once you are then logged into the “on the ship” version of the app, you can book things again. So, I would think this is the best way to book anything that has become available. I do t think guest services has anything additional to that. You can usually log into the ships WiFi from the port even before you board the ship.