So, on a previous trip, we went to Petrified Forest National Monument and Painted Deseret. That was actually really cool, and great break for a hour or two on a long drive.
(Yeah, I'm sure there is more than a hour to do there, but hit a few short hikes).

We also drove through Joshua Tree National Park (not on I-40, it's on I-10, but similar idea).... That was kind of fun too.

Anyone have any suggestions for Hidden Gems along I-40.
What about Mojave national preserve? Is it a "short easy hike" kind of place?

Something interesting for a quick break on the trip in or around Flagstaff or Kingman?