Capulin Volcano is a couple hundred miles away. That's practically a hop-skip-and-jump in west Texas. https://www.google.com/maps/place/Capulin+Volcano+Visitor+Center/@36.7787507,-103.9823705,17z https://www.nps.gov/cavo/index.htm
I'll mention this to them. They told me something about avoiding New Mexico because of covid restrictions, but I don't know the details.
Aztec Ruins National Monument is a day or two's drive from Houston. The Million Dollar Highway in Colorado is a quick hop from there.
As others have said, that is a large area. Not sure if it is closed or otherwise messed up because of Covid, but Johnson Space Center is pretty cool. Not exactly in Houston, but close by. And of course, lots of good food (again depending on Covid). Goode Company, Ninfa's, Otto's spring to mind as "oddballs" but tons of others.
The boys are still wandering around the country. They took some of y'alls advice and visited volcanoes in New Mexico and Monument valley in Utah. They are really enjoying the trip. I think they are doing the million dollar highway today. Good destinations in Colorado and beyond?