Although all of these interesting destinations are readily reachable from Oaxaca City [1], they are far too rich to be visited in a single day.
At a minimum, be sure to visit Monte Albán [2], which can be conveniently combined with a side trip to Atzompa [3].
On another day, preferably a Wednesday, visit the Etla market [4], with stops at the community museums and key archaeological sites at El Mogote [5] and Suchilquitongo [6] nearby.
Links:
[1] http://www.moon.com/destinations/oaxaca/oaxaca-city
[2] http://www.moon.com/destinations/oaxaca/the-oaxaca-valley/west-the-archaeological-route/monte-alban
[3] http://www.moon.com/destinations/oaxaca/oaxaca-valley/west-archaeological-route/santa-maria-atzompa
[4] http://www.moon.com/destinations/oaxaca/oaxaca-valley/west-archaeological-route/etla
[5] http://www.moon.com/destinations/oaxaca/oaxaca-valley/west-archaeological-route/san-jose-el-mogote
[6] http://www.moon.com/destinations/oaxaca/oaxaca-valley/west-archaeological-route/suchilquitongo