I have, on numerous occasions, had to throw water on European's plans to visit all of the big sites in the US in one visit. Wanting to see the Grand Canyon, the Everglades and DC in the same visit is not terribly practical. My advice has been to pick a region and see everything there. Pick a different region of the US on your next visit.
What was throwing them off was a day-trip can drive across several European countries, but will only get you through a few states in the US.