We actually sent a day in Puebla before making our way down to Oaxaca, but regardless, there is not much to talk about regarding the trip.
Day 1: Monte Alban and Zocalo
Monte Alban is hard to miss, as the ruins are built atop a mountain that towers over the city. Compared to what is to come in this itinerary, I would say that Monte Alban is reasonable archeological site, but half the value of the place is the view. Oaxaca is built where two valleys converge, and the site is located "in the cup of the Y" meaning you can a fantasic view down all three valleys.
Oaxaca is considered by many to be cultural heartland and gastro-capital of Mexico, and the esaiest place to experience this is at the main square. Musicians rove between restaurants playing mariachi music for tips, and the Benito Juarez market to the south has just about anything you could possibly want to buy. And if, for some reason, you can't find your dream souvenir, Oaxaca has numerous markets worth visit, including weekly not-exactly-farmer's markets.
Day 2: Hierve el Agua and Mitla
First thing's first, Hierve el Agua is on native land, and can only be accessed via tour companies (of which you'll find plenty near the square). But it is so, so worth it. Hierve el Agua is mountain spring on the edge of a cliff with absurd views any millionaire's infinity pool would die for. Just be prepared for a bumpy ride up and down the mountain to get there.
On the way back from Hierve el Agua, you'll invariably pass through the town of Mitla. Known as one of Mexico's most magical cities, the town has a number of Mezcal distilleries, a well preserved archeological site, and many artisanal stores where you'll get better deals on local crafts than in central Oaxaca.
Day 3: More Markets, More Food
Oaxaca really is the best place to pick up souvenirs and try out all kinds of foods. If possible, I would highly recommend rearranging this itinerary so that you can check out one of the daily markets. If nothing else, it was worth it for me to have a more relaxing day in preparation for the overnight bus that was to come.
It is possible to fly from Oxaca to Tuxtla-Gutierrez, from where you can bus to san Cristobal de Las Casas. However, being the cheapo backpacker I am, I chose the overnight bus, which had to deal with a very curvy mountain pass and uneven asphalt. If I were to do it again, I'd probably fly if the prices were decent.