Dropped off my bag at Hostal Naba Nandoo (where I'll spend the rest 2 nights. I highly recommend it for budge travelers), went to the center. 
The San Domingo church is the highlight. Bought a tour at the zocalo for M$200 from Sicaru Agency to Hierve El Agua and Mitla. I don't recommend this agency. It's a shopping tour. Too big a group: 2 vans, one 20p, one 14p. Unbeknownst to me, here, entrance fee is not included. The bilingual guide is nice, but totally useless. His English is as bad as my Spanish. Once he found out that we have a group of German kids, he switched to German instead of English. Not sure how bad his German is. 2 French girls complained. Also, he doesn't know much. I learned more from trailing other guides. The adjective he likes to use is "special". Everything is special. 
Our first stop, the 2000+ year old Ahuehuete tree in Santa María Del Tule. M$10 to enter the fence, even though you can see the tree equally well from the outside. There's another one, maybe 1000 years old. Both look very healthy. I do recommend this stop. But if you are penny pinching, skip the entrance fee.
2nd stop El Ray de Matatlan, a mezcal shop. We were separated in two groups, in English or Spanish. We were shown the pit used to cook agave, the horse drawn (w/o horse) mill, and distill pot and wood burning slot. Then we tasted 4 mezcal: the 5 year old, the 1 year with a worm, the 6 month, and the 8 years. There're also flavored mezcal to try. Orange wedges with pepper salt was used to tame the taste.
Next a weaving cooperative that uses wool (from highlands) and natural colors from local plants, rocks and insects (cochineal for red). An 8 year boy and a maestro in his 50s stood by their looms Then we were driven to a restaurant near Mitla for a buffet lunch, M$150. Several of us chose not to partake. Luckily they have chairs in the shade to wait for the group.
At 2:40pm left the restaurant for Mitla. The guide collected M$70 entrance fee on the bus. This I'd recommend not to bother. The Spanish build their cathedral on the Mitla site, and pilfered the rocks and columns. So only half of the ruin is in the fee part. 
The good thing is that there's no vendors inside, and you can go down to 2 underground chamber, which were too small and claustrophobic. 
Mitla was a Zapotec funeral site. It's said to be sacred to the Zapotec people. It has many mosaic rock decoration. Near the Cathedral, I was pointed to some fine red drawing above door ways by another guide.
4pm, we finally left for Hierve el Agua. On the hwy ramp out of Mitla, the auto toll is M$19 for the vehicle. Due to the bumpy road, we only got there 45 minutes later. Again, our guide collected M$35 (two entry fees $10 & $25). We were given 1 hour. The view of the Oaxaca Valley is excellent. There are two calcified waterfalls, most people and the shops are next to the first one, where the big parking lot is. There are 2 pools (maybe artificial) where you can bath (too shallow to swim). I wore swimsuit and brought my dry bag, but didn't use them. 
One water source is fenced off. You can see water bubbling. 
There's a foot path connecting both above, and I met some Mexican walking below when I reached the 2nd fall. So you can walk a loop. I walked down the 2nd one, and back the way I came. I highly recommend going down to at least of the dry falls. Quite impressive to see it from below. On my way back, saw 5 guys scrambling up the first dry falls. The last 2 people had a bit of trouble. 
By the road, when walking back to the parking lot, I found a tiny hill with great view of the pools and the valley. I might the the only person who was back to the bus on time. So... as a punishment, I had to wait for others. Had time to look around the tourist infrastructure built for this natural wonder. Seems not well used. A semi circle concrete building, with water sinks (no water) outside, a large pool (no water), toilets (half closed, half for M$3).Back to the city ~8:30pm. Chilly already. Elevation here is around 5000'.
11/24, day 9, Saturday. Sunny, no clouds. 30°C/12°C (85°F/53°F). Followed everyone's advice, I went to Mt Albán on a Lescas shuttle $58/RT. 8:30/12pm is the first one available. Contrary to what I read online, I'm allowed to take any return shuttle (once an hour starting from noon). 3.5 hours is sufficient to check out everything, even if I eavesdropped various guides. The shuttle takes 20+ minutes, picking up passengers on the way. I sat with an Indian couple who were traveling in Mexico for a month, while the husband's US visa was renewed. Two of them has only one 0.5L bottle of water, no hats.
Monte Alban (M$70) is really on top of a mountain, excellent view of the city below. Another Zapotec site 2-800AD. Later occupied by Mixtec people. It's a big court, with higher temples on north and south ends, two structures in the middle and other lower building all along east and west (almost none allows climbing, other than the lower platform of the south temple and one higher temple of the north. I walked clockwise. When we arrived, there weren't many tourists yet, even though it was a Saturday. The north plaza is the most elaborate. It has its own plaza on the lower platform. Probably ceremonial center. A series dancing figures carved on rocks look interesting. The originals are in the museum. The museum is fairly small. But it offers shade and washroom. A good place to idle while waiting for the bus out.
Once back in town, I hailed a colectivo (M$15) to Cuilapan de Guerrero for Ex-monastery of Santiago Apostol. My driver called it convento. I really like this ruin. A small new church is built between the two walls of the old church, and was having a mass. Even though there were occasional visitors, this is a quiet place. Nice to sit under a tree to relax - which I needed: wasn't feeling well. At the back of the group in the shade of the ruined church and big trees is where Vicente Guerrero was executed. 
It was still early. I hailed another colectivo to Zaachila. M$10. For the life of me, I couldn't remember why I marked it on my map. The central square looks quite nice. In the middle is a long stall of vendors under the same long roof. Most of them selling liquados. Nice for the hot day. Walked to the ruin noted on Google map. But seems like a wrong entrance. When I ended up is a fence in an alley east side of the ruin with dogs barking and chasing me out. Too hot to investigate the other sides. Went back to Oaxaca on yet another colectivo. M$15. There's a central depot for the colectivos here, where you go and wait for the next one to leave. Direction is "central".Back in the town, I went to the airport shuttle near Zocalo to book a pickup for the next day, but was informed that they didn't have driver that Sunday, so no service. Gorge at the hostel told me how to take a colectivo, since I had little luggage.
11/25, day 10, Sunday. 10am Free Walking Tour. We met in front of the cathedral. You can reserve it, and because of that, we waited for more than 10 minutes. The tour moved their starting point in recent years from in front of the government building to the current location (1 block north) due to the Solidarity Protest starting from 2006's demonstration of teacher's trade union.
We walked to Culture Center of San Pablo, converted/expanded from the the oldest convent in town. Then the public library named after Margarita Maza - wife of Benito Juarez, a Oaxacan. Then Contemporary Art Museum next door. When I left their clean toilet, I lost the group. There was a mining right demonstration walking by, quite a lot of people of all ages. I found the group at San Domingo. Then, we spent sometime in the organic market of Cosecha, with a big whimsical insect sculpture at its front door. There're about a dozen different stalls selling farm product. I had a drink. 
The street by the ancient Xochimilco Aqueduct is quite lovely and peaceful. Restaurants hid in the side alley under those arches. Our guide is a young girl. She also suggested some restaurants, but I only remembered one called Cabuche. 
Thanks to her, at noon, I took leave of the tour, and went to Teatro Macedonio Alcala, named after violinist Macedonio Alcala. There were a few locals waiting at its front door for free entrance. 10 minuets later, a lady let us in from a side door, and directed us up to the top balcony. 
I don't know the name of the first three movement piano concerto. Yes, the pianist gave an encore. After the intermission, it was Swan Lake. However, I didn't stay till the end: had to go to the airport. Certainly a better than expected finale of my trip.I walked to Hwy-175 south of the busy bridge (Periferico + 175), waited for a colectivo signed for San Juan Bautista la Raya. Had to wait for a few minutes (less than 10). The ride is about 10-15 minutes. M$15. I was dropped off at the entrance of the airport. From there, it's a 5 minute walk to the departure hall.
The flight was delayed for 30 minutes, even though the display at the airport showed on time all the while. My original connection at Mexico City was only 50 minutes, so I was worried enough to talk to a stewardess. After we landed at Mexico, we were taxing around a circle, before parked. The passengers in the first class were nice to let me get off the plane first. The airport crew sent a driver to pick up me, a guy for NYC, and 2 more to Portland, and drove us to our gate. Thankfully we didn't have to go through security scan again. A security guard in front of the departure gate asked me some questions and let me on my flight. All is well. I was back home around mid-night.
