When you say downtown, I guess you mean the financial district? I don't know of any there. There are alot of good spa's more towards the Galleria, could you take a taxi? Easy pickup both going and returning. I personally like the spas on the north side ( FM 1960), and the west side ( Chinatown), but those are too far for you without a car. You could take the light rail to the Medical Center, and go to Medical Center Spa, it's got 19 reviews, mostly good, never been though, although it is on my list. Ahhh too many spas, not enough time (or money)
I live in the Midtown area, and I also enjoy SUNFLOWERS the best. I think they have the best looking girls for that area and have a clean and discrete location that I like.