Driving through Arizona on the 40 there is a crater you can stop and see. It is not as out of the way as some of them places and it's not usually really crowded since you are in the middle of nowhere.
The only other thing I would suggest is to make an activity book for the boredom times. I made a binder for my daughter that was Disney themed when we flew to Florida. It had coloring pages, game pages, word searches, tic tac toe, all Disney themed. I saw some road trip scavenger hunt stuff online when I was building the book, but since we were flying I didn't save them. You can print everything out and put it in a binder. Just add a pencil pouch with twist able crayons that don't melt so easily or markers and pencils.