Brick
Starting from Martian clay, a brick-maker can create a wide variety of structures and paved surfaces as well as furnaces and ovens for smelting, blacksmithing, glass-blowing, cooking, etc. The brick-making craft requires only other small-scale crafts for its equipment (blacksmithing for its iron tools), thus qualifies as a small-scale craft suitable for a frontier town (small and largely self-sufficient) economy.
Due to the 1/3 gravity, structures made out of brick can be much larger than on Earth, yet still hold up under their own weight and be easy to transport.
