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I will suggest you consider Red Rock, which is very close Vegas, and Valley Of Fire, which is also fairly close. Needing to be back by 5 PM means you do not want to go too far. Ideally you want to get out early for before and shortly after sunrise if you can.
The road in to Red Rock does not open until after sunrise when the park opens, but there are some possible early in the day shots from near the road that goes South of the Red Rock entrance that you can try. It is not real common, but if a sunrise "pops" nicely with great red light, the whole range of the hills called Red Rock can be very nice.
Valley Of Fire is a larger area with more variety of subjects, and you can go in on the roads at any hour of the day or night. But it takes well over an hour to get there from Vegas, and you would want to know where you are going in order to best catch early light.
I suggest you keep in mind the value of a good polarizer for almost every shot you may consider in the Southwest. Besides cutting distant haze, you can also improve color saturation of the red rocks even if close in, because they have so much sand in them that reflects in a specular way. Take the time to view the effects as you adjust the filter, and you can easily see the improvements.