Water is always good to bring as it can be quite warm during the day. Sunscreen is good as well as a hat. Several of the days we will be out at lunch time, so bring something to eat for those days. Be prepared for hot as well as cold. Early morning can be cold. Bring a tripod for the early morning and evening shots as well as a remote release. Bring a map or GPS. Triple A has a good map of Death Valley. Apparently cell phone coverage is sparse, so don't depend upon it working.
We are going to bring those tuna lunch packs. Since everything is in a sealed pack, you do not have to worry about spoilage or keeping them cold. The idea would be something that does not need to be kept cold or to be reheated.