Favorite thing: When entering Piedras Negras you are not checked for your identification and very very rarely there is a customs check. The problem is getting back to the USA, there is ID check and they have tightened up on that. I would take at least 2 govt issued IDs. I cross this border post often enough, since I am not american, i am always questioned but the people are polite and professional at the USA border post. Enjoy Piedras, the food there is way better than in Eagle Pass.
Favorite thing: Piedra Negras is a city of close to a quarter of a million people, yet in the city center one has the sense of being in a much smaller town. The largest and most impressive building in Piedra Negras is the City Hall, across the street and to the west of Gran Plaza de los Heroes. Traffic in front of City Hall was fairly heavy, and I had to stand and wait for several minutes to get this clear photo of the building.