Inspired by @7829V 's reporting, I thought I would share my experience at Mr. TJ's Only Boys Spa in Zona Centro yesterday; it was a delightful way to spend a part of the day.
I took the San Diego Trolley to the end of the line at San Ysidro, walked across the border and took an Uber to the Zona Centro location. I had made an appointment for 11 AM, but arrived maybe 25 minutes earlier than that; the entry was was shuttered but in several minutes a young man arrived to open the spa for the day. When he had done so, I told him I had requested a Turkish bath and a 60-minute massage; I'm not certain if he was aware of that, but it was fine with him, and I paid 2,000 pesos ($113) with my credit card.
The young man, Yahir, guided me to the massage room to change for the Turkish bath; he gave me a towel to wrap around my waist and a pair of sandals; that bath itself was luxurious; water, soap, scrubbing, repeat. After a shower, we went back to the massage room; where Yahir, now nude, gave me what I thought was a good therapeutic massage with wonderful erotic touches (I couldn't help but grin at the time and I am grinning now as I write this!). The session did turn into a deli experience; Yahir was so sweet that I wanted to adopt him and bring him home! I had planned on tipping him $40, but I was so happy with the session that I gave him $60; he gave me a big smile.
Yahir, who I later learned was 25, had better skills in English than I had in Spanish, but that did not detract from my session in the least.
After lunch at Caesar's Restaurante on Av. Revolucion, I walked around a bit with my camera and, since I was dreading Friday afternoon lines going back to San Diego, I walked back to the port of entry, paid $20 for a bus to take me across the border, went through customs and took the trolley back to San Diego.
Yahir and I were alone the whole time, and that was fine; the place, as indicated above, is not fancy, but it is clean and I felt comfortable the whole time.
I'd encourage those of you who are interested in an adventure to consider this if you are visiting San Diego. (I am still grinning!)