One or more days (easy day trip) at Joshua Tree NP - an hour from Palm Springs. Really worth it.
Hoover Dam is easy from Las Vegas - short drive. The Grand Canyon, not so much - takes a good 4+ hours on the road. Ditto Bryce Canyon.
What about Zion NP instead of Bryce? You could do a loop - LV to Zion to Grand Canyon to LV with around 12 hours wheels turning, so doable with three days - 1 to Zion, 2 to GC, 3 to LV, with time in each day to actually see things.