Death Valley is interesting because of the numbers: elevation extremes, high temperatures, and so on. But visually it may not be as striking to the Dutch as the Grand Canyon. Also, your friends will not be acclimated to the kind of heat you may encounter in Death Valley (even in September). For those reasons, I would go east.
I don't know what your budget is, but you might even take them to the Skyway:
It isn't cheap, because it comes as a package deal through the Hualapai Tribe, but that would be a striking contrast to anything in Holland.
You might also take them over the Glen Canyon Bridge, for a cheaper thrill of heights: