Las Manitas has very traditional Tex-Mex food and a great atmosphere. It has the feel of a diner but there is a large screened-in patio in the back (you walk through the kitchen) that feels more like a garden room.
The food is priced reasonably but they do charge for chips and salsa which is unusual in Austin. Maybe it is because it is downtown???
I particularly like their verde enchiladas - YUM.
Try to get there early to beat the lunch rush or check them out for breakfast.
It is hard to find a good, nice Mexican food restaurant downtown, but Las Manitas fits the bill.