Absolutely stunning! Sunset time would have the best view but any time when blue skies are out is just as amazing.
It's choose your own adventure: stay in the car and drive, hike, bike, or explore by foot. You could pack a lunch and picnic, which I wish I had done. There are tours available through 3rd party companies but I'm so glad we saved the $$ and only spent $7 bucks and rented a car. The ticket is good for 24 hours with re-entry.
*tip: if you fly in early to Vegas and need time before hotel checkin, this is a perfect time to kill. Down the street are places to eat but nothing once you pay the $7 fee.