From pages 9-10 of Oaxaca Tips
Museum of Oaxacan Cultures and Santo Domingo Cultural Center 
M.Alcalá (next to Santo Domingo Church), Centro Histórico
Daily except Mon: 10AM-6PM.  Entry $51.   Tel:516-2991
Audiophones in English ($50) or Spanish ($40)
Housed in the splendidly restored Dominican monastery of Santo Domingo de Guzmán, the Museum of Oaxacan Cultures houses the region's main archaeological, ethnographi, and historical collections, including the Mixtec treasures from Tomb 7 at Monte Albán. This is Oaxaca's foremost museum and not to be missed. Program several hours to see it all, and take time to enjoy the magnificent views of the city and the adjacent Ethnobotanic Garden from the windows. Labels in Spanish only.
The up-market museum shop off the main cloister sells art and archaeology books, jewelry, clothing and folk art from various regions of Mexico.

Santo Domingo Cultural Center:
presents art exhibits and occasional concerts.

Francisco de Burgoa Library: Also part of the complex is this splendid library containing books and documents dating back as early as 1484. The core of the collection comprises tomes from the colonial monasteries and convents of Oaxaca expropriated in the 1860s. The library can be used only by researchers, but temporary exhibits with a historical or cultural focus are featured in the ante room.

From page 7 of Oaxaca Tips
Ethnobotanic Garden of Oaxaca
Corner of Reforma and Constitución, Centro Histórico
Entrance by guided tour only.
   One-hour Spanish tours: Mon-Sat: 10AM, 12PM, 5PM. $ 50
   Two-hour English tours: Tue, Thur, Sat: 11AM.  $100
Housed in the former field area of the Santo Domingo monastery, the superb Ethnobotanic Garden displays an extensive collection of native plants of the state of Oaxaca. Whether you count yourself a plant-lover or not, the Garden is an enchanting place to visit and offers visitors an unparalleled opportunity to learn about the extraordinary biodiversity of the state and the cultural and botanical importance of its flora.

There is often little shade: bring sunscreen, a sun hat, and water.

The adjoining Ethnobotanic Library houses a sizeable collection of books about plants and gardens, many in English, and is open to the public. Mon-Fri: 9AM-7PM; Sat: 9AM-1PM.   Tel:516-9017