Alright here are a bunch of budget options, everything pretty much under $20 and they will all make a significant improvement on your Coachella experience.

Earplugs : Personally I have these and I really like them. Only $17 and they save my ears in Sahara and the smaller tents.

Plastic flask : Definitely don't bring liquor into the venue with these $10 plastic flasks and mix it with the lemonades you buy. Definitely don't do that.

Camp sink : Its just a tupperware container that breaks down/expands so you can wash dishes in it. It sounds dumb but its nice to use something specific that doesn't take up any room. $9.99