"The ancient Mesoamerican pyramids of Teotihuacán, in the Valley of Mexico, once served as the largest city in the pre-Columbian Americas. It is thought that during the first millennium A.D. the city had around 125,000 people, including multi-ethnic groups such as the Otomi, Zapotec, Mixtec, Maya, and Nahua. If you have a few days in Mexico City, it's worth the day trip. (Teotihuacán is about an hour outside the city by car.) Leave early in the morning so you can be back in the city by mid-day—and bring sunscreen!" - Scarlett Lindeman