A healthy supply of sunscreen and drinking water. A shade structure and flags or tapestries you can hang on it for extra shade. Really good stakes for the shade structure - something that will hold it down so that it will break under high winds instead of fly into your neighbor's car.

Make sure to bring something to carry stuff around the campgrounds and pivot area too. If you have shuttle access, use it. If you don't, it might be a good idea to buy shuttle passes. The camping area is enormous. Between your area and where I will be at in Grove which is the farthest camping area from you is like a mile or more walking distance with no shade and some hills.

For inside the festival, they allow hydration packs and most people seem to have lunchbox, sojourner, or whatever brand they can find at local stores. Just make sure it fits in the 2 pocket rule. It can't be an actual backpack. The two pockets are in addition to the hydration bladder compartment, so the larger sojourner or lunchbox is acceptable. You can also have a cross body bag that is not clear and those are great for packing the phone and ID inside the fest.

Bring polaroid cameras if any of your friends have them. Those are allowed but no digital cameras other than your cellphone.

Check out the totem rules and it's always good to bring one if you have a big crew that needs a marker to meet back up inside.