Here is my secret list shhhhhh... It's not cheap options that's for sure but I like being comfortable I guess, and it's stuff that will last for years of festival fun!
A deep cell marine battery: Available at any auto parts store for about $120. You can then buy a cheap cigarette lighter hook up and intall it onto the battery. Then get yourself a decent 200watt power inverter, about $30. Then you can get some decent sounding speakers that plug in or a bluetooth one. Then you can charge your phone, power the speakers, or charge the bluetooth. With these speakers the battery will last 16 hours! This will also prevent you from ever messing with your car stereo or battery for any reason. Then you can just recharge it with this for next year.
Blackstone's The Dash Portable Gas Grill and Griddle Combo is a great grill to cook every meal you would ever need.
Not to get controversial but these and these have always done me right.
Then this and this will leave you fealing like a million bucks or like someone in GL!