Apart from the usual like tent, sleeping bag, phone, money.
A smallish square of foldable but fairly tough plastic that you can keep in a pocket so you can sit on the ground without getting a damp backside
Decent and broken in walking boots, forget wellies, rain gear - trousers and jacket, you'll look like a gobshite but be nice and dry underneath, can be quickly removed if rain stops or heading into tent for a gig.
Stay in Boutique camping, once you experienced it you will never go back to general camping.
Earplugs, blindfold, camp bed or decent self inflating mat, decent hiking boots, foldable water container (holds a gallon so only one trip to the taps each day, if it gets too hot, hey presto - a shower), BREATHABLE waterproof jacket - forget about the plastic ponchos you will just end up soaked in your own sweat, shorts, socks and jocks, t-shirts, a decent hoodie, sunglasses, sun block, micro fibre towel (they dry in no time if laid flat), multipack of toothbrushes in case you drop one, TP/Baby wipes, beer, buckfast, spirits along with a backpack for same, and a decent camping trolley to put it all in - Never get taken to the cleaners by boutique camping again!
...seeing as I'm now mid 30's unfortunately 5-HTP, Berocca, Dioralyte and pain medication are now also required!