Museo del Fuerte de Loreto
A five-minute drive north of the Zócalo in an otherwise nondescript industrial area, you'll find this excellent museum inside the Zona Histórica de los Fuertes, an expansive park that commemorates the city's colorful military history, including the famous Battle of Puebla between France and the nascent Mexican Republic, in which the latter prevailed against what was considered to be far more formidable force. Unless you're a big history buff, you may not be familiar with this battle, but you probably recognize the date: May 5, 1862, or Cinco de Mayo, which though a popular holiday outside Mexico is a far less important date than that of Mexican Independence Day, on September 16. Nevertheless, the museum here inside this well-preserved fort is one of the more interesting, and underrated, ones in the city; it's filled with exhibits and artifacts that tell the story of this battle. Across the park, Museo del Fuerte de Guadalupe is interesting as well but not quite as comprehensive. It's worth a look, though, if you have time.