Many tourist attractions now offer online and advance reservations. These make great sense if you are planning to visit a very popular attraction. Otherwise, you could spend a large part of your day waiting in line, or miss out altogether for sights which limit the number of daily visitors. The down side of this travel tip is that you sacrifice some scheduling flexibility, but, to me, it is worth it. Our reservations at the Vatican Museum allowed us to bypass the 2-hour queue. On the other hand, my sister was unable to see the Alhambra because she arrived at the site after the maximum (controlled) number of visitors had been reached, without a reservation..