How do I plan my Las Vegas visit?

Hi all.
I’m visiting Las Vegas with my SO. Both of us are 28 years old, no kids, coming from Asia.

We’ll arrive in late September, having a night + 5 days + a morning in Vegas. I really want to take a car, and experience the nature around Vegas. I don’t mind being left with 2 days or so in Vegas. Note that I can’t shorten or extend my stay. I arrive at Vegas airport and leave from the same airport.

What would you suggest? Of course this includes staying around the parks. Ideally I’d like to arrive at Vegas, have a day there, see parks for 3 days, and end my stay in Vegas with another day.

Also how would the weather be in those parks in late September?

Thank you so much in advance!