Regardless of whether you intend to book your sightseeing through the cruise line or do it on your own, you want to figure out what you want to see well in advance and make the necessary arrangements. See p.102 for a list of outside companies that handle these arrangements, sometimes at a lesser cost. If you do decide to book tours through your cruise line, you will find plenty of details on their website about your options.
Many lines allow you to book there as well.
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The timing varies, though: Some let you book months out, while others are mere weeks. (And the closing dates vary, too. ) Just be warned: Your first choice could sell out before you board if you don't plan in advance.