Minimalist list of things to see/do in Death Valley:
Artist's Palette
Badwater
Sand dunes near Stovepipe Wells
Scotty's Castle
Ubehebe Crater
This is a BIG park. It is about a 90 minute drive from Badwater to Ubehebe Crater. This is a very busy full day if you include the Scotty's Castle tour, which is worth doing. You cannot drive in from Las Vegas on day X and drive somewhere else on day X+1 and fit in just these few things.
Other things well worth doing include Zabriskie Point, Golden Canyon nature walk, Dante's View, and Harmony Borax Works/Mustard Canyon. Also a splurge dinner at Furnace Creek Inn -- you don't have to be staying there to reserve and eat in the restaurant -- is worth considering, the food has really improved in the past several years.
If your rental car has reasonable ground clearance, you can pack a picnic lunch and go do a day trip to Rhyolite and Titus Canyon. This is a full day in itself and truly spectacular although you need a driver who is used to sometimes psychologically daunting roads. Drive the few miles further at the start to Beatty, Nevada, to get gas, it's about 80 cents a gallon cheaper than at Furnace Creek.
On the other hand, maybe the rental car doesn't need reasonable ground clearance. Do you know the difference between an expedition equipped Land Rover and a rental car? You can take a rental car
anywhere! 