For years, when we traveled, my kids would make a chart of all of the states, then check them off when they found the license plate. (We once found an Alaska plate in Florida!) Sometimes they would bet a candy bar for who would finish or get the most, first. Might be fun for the youngest.
Share what YOU know about different areas as you travel. Explain to them how to read maps. Play estimation games. How many miles do you think we have gone? Calculate hours of travel. Make lists of things of interest, to look up later.
Maybe the kids would like to make their own US map, and add points of interest as they go?