For campsite music I can't recommend this thing enough. The most powerful portable speaker I could find and runs on 8-D batteries. Was $350 last year, down to $230 now:

http://www.amazon.com/Klipsch-KMC-Wireless-System-Bluetooth/dp/B00BAQJ94Q/ref=sr_1_1?ie=UTF8&qid=1420310784&sr=8-1&keywords=klipsch+kmc+3

Coleman EZ-UP, I realize this one's is on the more expensive side but in my experience Coleman products are really great at being truly waterproof, no need for extra tarps and this one has LED lighting built in so no need for lanterns, etc:

http://www.amazon.com/Coleman-Instant-Canopy-Lighting-System/dp/B004E4CUBK/ref=sr_1_1?ie=UTF8&qid=1420310839&sr=8-1&keywords=coleman+canopy

Sanuk shoes with yoga mat lining. Especially crucial on days 3 and 4:

http://www.amazon.com/Sanuk-Mens-Vagabond-Yogi-LoaferCharcoal10/dp/B0089YOAZK/ref=sr_1_1?ie=UTF8&qid=1420310892&sr=8-1&keywords=sanuk+vagabond+yogi

Portable Battery for cell phone charging with tons of juice for several phone fill ups:

http://www.amazon.com/KMAX-810-20000mAh-Ultra-High-Multi-Output-BlackBerry/dp/B00FDER07E/ref=sr_1_11?ie=UTF8&qid=1420310945&sr=8-11&keywords=portable+battery+20000mah

Hangover assist/vitamins/electrolytes/hydration:

http://www.amazon.com/gp/product/B00EISFBYA/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

That's all I can think of for now, it really is great being fully prepared for this event :)

Posted this back in the great amazon thread, should have plenty of refills on this baby:

http://www.amazon.com/KMAX-810-20000mAh-Ultra-High-Multi-Output-BlackBerry/dp/B00FDER07E/ref=sr_1_11?ie=UTF8&qid=1420310945&sr=8-11&keywords=portable+battery+20000mah

Yes! Check out our Great Amazon Thread! Or please hold I will provide a direct link. Thank you.

EDIT: Most bang for your buck resides here