Cerro de las Culebras (Snake Hill; Coatepec in the Náhuatl language) is easily accessible from the town center. The walk takes you up cobbled steps to a lookout tower with a white statue of Christ on top. From here there are magnificent city and mountain views (it's best to go in the morning).
To get there, walk three blocks west from the main plaza on Lerdo, then north all the way up Independencia.