We usually book the excursions a little more than a month before we sail. That's usually about the time our final payment is due. We've already checked them out before that and then we narrow down our decisions. Since some of them can be popular, we figure that we might as well book early to make sure we get the times we want. Given the long lines at the Excursion Desk onboard, why have to deal with it if they provide you with options to not have to worry about lines?