If Taqueria Aranda's is still open on Military Drive, I would recommend it for lunch. I haven't been out there in about 15 years but the building looked like it needed to be condemned, which is a sign of good tex-mex in SA. Taco Garage on Broadway is decent. If you get up towards Walzem, Cristan's Tacos at Walzem and Midcrown has excellent Carne Asada tacos, but they close at 2pm everyday and they are closed all day on Monday. If a sit down restauarant is more what you are looking for, I would go to Mamacita's on I-10 in the Medical Center before dealing with tourists at Mi Tierra, but I try my best to avoid downtown. Just stay away from Taco Cabana and Las Palapas, they are fast food chains, better than Taco Bell, but that isn't saying much. Mama Margie's if one is nearby isn't bad, better than the two chain places.
For BBQ, try the Smoke Shack on Broadway, or go to Texas Pride on the far east side. A lot closer to Lackland than Luling or Lockhart. Fair warning, Smoke Shack has a small parking lot and can get downright packed. Texas Pride gets busy during lunch, but is fairly calm around 3 - 4 pm on weekdays. Harmon's in Marion/Cibolo is pretty good too.