Long socks for under the wellies (if you're wearing with shorts/skirt). Welly rash is awful!
A hat for if it's sunny as there's practically no shade there!
Warm clothes/layers for sleeping in. I get FREEZING at night there and wear big thick socks, tracksuit bottoms, long sleeved top, thin jumper and a hoody at night (sounds like overkill I know!)
Hoody/jumper for putting on I evenings, waterproof jacket, a few changes of socks/ leggings etc in case of mud.
Whatever outfits you like! People wear all sorts from super practical to fashion parade so anything goes as long as your comfy.
Baby wipes, hand sanitiser, packets of tissues (for the loos), sun cream, cereal bars etc etc. Bin bags (for rubbish at camp etc), mints or chewing gum, dry shampoo etc.
Have fun!! Didn't get a ticket this year (sob).