Cholula is the old city just west of Puebla, now folded into the metropolitan area but still distinct in character. It is famous for one astonishing image: the Great Pyramid of Cholula, mostly hidden under a grassy hill, crowned by the yellow Iglesia de Nuestra Señora de los Remedios and backed on clear days by Popocatépetl.
The city is really two neighbouring municipalities, San Pedro and San Andrés, with pre-Hispanic, colonial, university, and bar-going Cholula all layered into a compact area. Around the Plaza de la Concordia, San Pedro feels traditional and civic; around San Andrés and UDLAP it feels younger and more nocturnal.
Cholula works beautifully as a day trip from Puebla, but staying overnight changes the rhythm. The tour groups leave, the pyramid hill glows in late light, and the restaurants and bars around San Andrés fill with students. Add the San Gabriel Convent, the Regional Museum of Cholula, and the extravagant church at Santa María Tonantzintla and you have one of central Mexico's easiest, richest side trips.