Don't even think about visiting Death Valley National Monument without having an airconditioned car! The temperature at Death Valley very seldom drops below 90 degrees Fahrenheit, not even at night. At least not in summer.
So the best way to get some expressions of that truely desert place is to drive through it. There are a lot of viewpoints along the way, from where you can see bizzare formations like the ones from Zabriskie point.
If you wanna stand at the lowest point of the United States, Badwater, with an elevation of -252 ft., you'll need to step out of the car and take a little walk downhill. Another interesting feature are the Sand Dunes along the way...check it out!

I would also NOT recommend to spend a night at one of the campsites in the park. The temperature barely drops at night and there is no cold water available, although there are signs on the fresh water facilities, indicating so.
So if you really have to spend a night here, rent a room!