Oaxaca's Casa de la Ciudad is housed in a big yellow building just a couple of blocks from the Zocalo. On the ground level you'll find the Andres Henestrosa memorial library which contains over 50,000 volumes, and some rooms that are used for temporary exhibits. Make your way across the central patio and up the steps to the second floor, where you'll find, among other things, a room that has two very large aerial photos (about 12 square feet) of Oaxaca city on the floor. One of the photos was taken in 1990, and the other in 2006. They call this the "foto-piso" (photo-floor). It's fun to walk over it and pick out landmarks and see how Oaxaca has changed over time.
The Casa de la Ciudad often has exhibits dealing with urbanization and architecture, and it also hosts workshops, concerts and other events. It is open daily from 9 am to 8 pm, and admission is free.