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I would suggest you prebook any trips you fancy as it is very frustrating when they sell out, you can always cancel provided you give 24 hours notice.
There will be some availability for many of the excursions but this cannot be guaranteed. Popular excursions like tree top walks often sell out. I would recommend that in your case you book the excursion(s) you would really like to do and take a chance on the others remembering that you might well pal up with another couple who are more experienced and be able to suggest things that you can do together at a much smaller cost (and more fun). All this fuss about missing the ship...it so rarely happens and invariably it is the fault of those who should have planned better. .... Neil.