56 L of water for drinking, washing, whatever, with portable bottles for toting around (enough for say 7 days, at 8 L/day)
One ticket to get into a festival or event.
About $1200 US cash, and a credit card. Conservatively allowing about $100 Canadian per day in the desert for food and whatever you might need to pay for, if you find yourself needing to pay cash for something during a week's stay, C$100 per day during your trip to and from the desert, which you also assume might take a total of about a week, and finally, about US$300, assuming you forgot your ticket to a festival or event, and you find you need to purchase another one at about twice what you had to pay to buy one in advance.
Sleeping bag
Pillow
One Digital Camera, with extra battery, 12V adapter/charger, and sufficient image storage memory
One portable MiniDisc or DAT recorder, with plenty of tapes or discs, and plenty of batteries
One AT825 stereo microphone, with standard cable, XLR-pair to mini-stereo plug adapter cable, stand, and windscreen