Death Valley can get pretty hot in May, but it's still early enough that the higher elevations will be comfortable. Where you go will depend on what you want to shoot. May is too late for wildflowers on the valley floor, so if you're looking for flowers I'd recommend Wildrose Canyon or the road to Aguereberry Point. Birds will be concentrated around the springs (try Wildrose Canyon or Scotty's Castle), rabbits and other mammals will be most active in early morning or late afternoon, snakes will be out at night, and lizards will be everywhere, all day long. You can shoot landscapes and ghost towns any time, but you'll find the best light in early morning and late afternoon (just like anywhere else). There's way too much to see in one visit, so you should try to gather as much information and advice as you can in advance and decide on your priorities -- but stay flexible, because you never know what you'll find when you get there. Have a great trip!
Dan