Well first, read through your Coachella welcome guide that comes with your box. It has tons of awesome information on what to expect.

Then, use the search bar and find tons of other threads recommending camping items. Seriously there's so many, nobody will cover them all in this thread.

That being said, here's a copy/pasted comment of mine from another similar thread. These are some of the lesser known items I like to recommend:

"Individually Wrapped Butt Wipes: low profile enough to stash a couple in your fanny pack or backpack. Make the porta potties wayyy less of a nightmare.

Individual Pedialyte Packets: never worry about hydration. Just drop one into every water bottle or Camelbak refill. Also tastes amazing in conjunction with "goodies."

Chelated Magnesium: if your "goodies" make you queasy or clench your teeth a lot.

Hydro Flask Cup: It's going to be hawt as FUDGE out there. Last thing you want is your drink getting warm. I've left these on the hood of my car by accident and come back to it still full of ice. Amazing.

Ravpower Portable Charger: this bad boy is one of my favorite purchases ever, not just for Coachella. When this thing's fully charged, it charges a full phone battery around 6-7 times. It charges like twice as fast as my wall outlet charger. It's seriously amazing.

Individual toilet seat covers - Those camping porta potties are a fucking warzone sometimes.

I'd also recommend side walls for your EZ Up as extra shade. Tapestries work too. "