Westminster Abbey, the Eiffel Tower, the Sistine Chapel — these are the reasons you go to Europe. They're also the reason millions of other tourists go. Nothing kills a sightseeing buzz like being shoulder to shoulder with mobs of other visitors. At popular sights, it can help to arrive early or go late. This is especially true at places popular with cruise excursions and big-bus tour groups. At eight in the morning, a scenic walk atop the city walls of Dubrovnik, Croatia, is cool and easy, with