Fanny Pack - They don't look cheesy if you rock them with a fun style. They can fit everything you'll need for yourself, if someone else in the group has a backpack. Just be sure to wear the backpack sometimes so that people aren't annoyed by you not helping out

Black Collapsible Flasks - Easiest way I've found to sneak in booze. Plus, if security ever does find it, who gives a shit if they throw it out, since they're like $4 a pop. Red Bulls are only $4, so they make a great mixer/chaser Example - $7.75 for two-pack

Diffraction Glasses - Depending on what you'll, uh, be doing there

Industrial grade glowsticks and floss - They glow for a looong time. Every night, we gave everyone in our group a heavy duty glowstick and made a necklace/headband out of floss. This allowed anyone to be able to easily hold up a glowstick to help others in the group find them quicker. Plus, if food gets stuck in someone's teeth and you have floss on-hand, you're like a savior. Example

Good earbuds - Do not fucking skimp on earbuds. I know you said Amazon, but I'm partial to Earpeace. Regardless, don't go with the basic foam ones... there are plenty of good new brands out there making high quality, reusable earbuds for under $20 and they all come with a carrying case that'll fit right into your fanny pack

Single-use preservative-free eye drops - Dust gets in your fuckin eyes all day. Dust gets in your friends' eyes all day. Anyone wearing contacts will be especially uncomfortable. And security won't confiscate disposable eye drops (I'm not sure what their policy is on normal bottles). Just throw like 10 in your pack per day. You'll probably only use a couple, but they're small. Any of these will be fine

Bandanas - Multiple uses, especially for covering your mouth for the walk home. Wet the part that covers your mouth for extra protection from dust. Plus, you can use them for your glowstick headbands. If you want to be strictly utilitarian for the dust, hospital masks are awesome... especially if you're going weekend 2 (grass is dead, so it's dustier)

Pedialyte - Drink that shit every night

A lot of this stuff is cheaper if you buy in bulk, which is good for groups. So you don't have to be the one that buys everything. Or buy everything and have everyone split it, that's cool too.