A Slightly Different Take
With so much to see between central Colorado and Los Angeles, and a time constraint - even a week, I'd be inclined to keep the driving route as direct as possible and use more time in the available parks. Those parks would include Colorado National Monument, Arches National Park, Bryce Canyon National Park, Grand Canyon National Park (North Rim), Zion National, Red Rock Canyon National Conservation Area (outside Las Vegas), and Mojave National Preserve. Now Colorado National Monument, Red Rock Canyon and Mojave National Preserve might require less time than the rest, but in general they'll need to average about three hours a day of driving and seven to ten hours daily of actual sight-seeing. Sure, you wish they could spend more, but I think a once-over-lightly visit t as many different parks as possible would be best since it won't really change their total balance of driving/exploring time.
AZBuck
Don't think that will work.
Buck, this is in the second half of March. The North Rim (where I would love to send them) will not yet be open. By direct, I had figured something like I-70, UT-128, US-191, US163 and on to the east entry. But it is getting it right time wise, and I just am not good at that.
Just realised that I did not ask her if this is 6 nights or 7 nights. They are away on their Christmas vacation at present, but I will get that information when she returns next week.
Lifey