The historic core, pyramid, plaza, and San Gabriel Convent are walkable. San Andrés, UDLAP, and Tonantzintla are farther out, so use taxis, rideshares, or local buses if you are linking several stops.
Pavements can be uneven around the older streets, and the pyramid hill involves steps. Start early if you want clear volcano views before clouds gather around Popocatépetl.