The Santa Elena area near the Rio Grande was marked as a "must see" and our tour guide app Just Ahead mentioned it as well. We loved having this app which acted like a tour guide since it was connected to our map system and would point out features and areas to stop as we drove.
We caught a road runner running across the street, which was a big highlight the boys mention when people ask about the trip. They were pumped up after hiking up the switchbacks to get to the top of the canyon on the US side. Half way across the river is considered Mexico. J threw a rock across to Mexico, and how many opportunities do you have to say you are that close to the border. The walls of the canyon have shell fossils all over.