Do they have a 4WD high clearance vehicle? That will expand their options quite a bit. Pretty much anything with spectacular scenery that is easily accessibly by passenger car and not too hard of a hike is going to be crowded. Similarly, are they comfortable navigating off-trail (or at least where there are no official signs)?
Canyonlands is my favorite national park, but my favorite parts of it (the Needles and Maze) aren't the best for this kind of trip. Island in the Sky is worth seeing, though.
The primitive loop in Arches is one of my favorites. The first 40% (up to Double O arch) is super crowded but worth it anyway because it's awesome. Arches in general is nice for this kind of trip, especially if they can get one of the car camping sites there in the park (camping there is my favorite part of that park!)
A few days in the Robbers Roost area could be really fun. Horseshoe canyon (the best rock art panel in the country?), the non-technical part of Bluejohn canyon, Colonnade Arch, etc. I haven't been there myself but if they have an appropriate vehicle and sense of adventure, that area could be great.
The Swell also has some classic areas as well such as Goblin Valley, Little Wildhorse Canyon, etc.
I haven't been to Bryce yet but I think it would be nice for this kind of trip.
I don't think Buckskin Gulch would be as good for this. I haven't done it yet (arggh!), but I think I'd save that for when they can get deeper in to it, as my impression is that the best parts are further than 4 miles in. Even though the Zion narrows is way too crowded, it might actually be better for a short in-and-out.
Check out some of the hikes around Cedar Mesa such as the Citadel as well