Mexico city
The Zocalo in Mexico City is one of the most vibrant places I've traveled to, in one of the largest cities in the world. There are so many cool things to see aside from just getting lost in the meandering side streets of markets, artisans, museums. Check out Zona Rosa, Poblano, Chapultepec neighborhoods. the Mayan exhibit in archeological museum is brilliant. Easy place to stay is hotel mayaland on Reforma/insurgentes. Easy ride from airport, makes the initial stress of new city a breeze. Hemingway wrote about it, Kerouac immersed in it... definitely a city to spend some time in at least once!!