The main post office of Mexico City — opened in 1907 by President Porfirio Díaz — is a working post office that doubles as one of the most spectacular Beaux-Arts interiors in the Americas. Adamo Boari designed it as part of the same project that produced the Palacio de Bellas Artes across the road, and the building is sheathed in cream Italian limestone, with a central hall of Mexican onyx, white marble, and gilded bronze.
The double-helix staircase, the iron-and-bronze counter screen, and the painted glass ceiling all survive. You can still buy stamps and post a letter here — the building was lightly restored in the 1990s after damage from the 1985 earthquake. A small philatelic museum upstairs is free; entry to the main hall is also free.