Holland America Line attracts scores of devoted fans who love the grown-up-focused experience on board. Entertainment centers on what a mainstream, slightly older crowd would enjoy: The line has hosted “Dancing With the Stars” celebrities and musicians straight from B.B. King’s nightclub in Memphis, Tennessee, and a partnership with Manhattan’s Lincoln Center brings classical musicians on board the line’s newest ship, Koningsdam. Passengers appreciate the little extras, too, from complimentary hot appetizers during the very much observed predinner cocktail hour — at bars all over the ship — to free tote bags for all passengers.