Tacos in Downtown

I have previously been to Houston with my father and he is now going back alone. During my exploration I ate 30+ tacos in a 4-day trip, so he asked me for recommendations that are in or walking distance from downtown only. In that area I have been to La Calle and Tacos a Go Go, and liked them well enough. Does anybody have any other recommendations for good, proper tacos? Open to anything – stands, trucks, restaurants, whatever – as long as it’s near downtown