Read the list of forbidden items, traditionally there’ll be at least one thing that’ll catch you out. For example, Reading festival, not allowed to take deodorant. You can buy it once you’re inside, you just can’t take any in.
Mine is like this:
Tent
sleeping bag
air mat
extra fleecy blanky that I bought at about 4am at Reading one night to fend off death
Wee first aid kit of convenience things- there’ll be first aid available but blister plasters, stuff like this.
Wet wipes and some disposable cleaning stuff
BOG ROLL.
Sun cream
Waterproofs- if you’ve got a pocketable bike waterproof, ideal. Lots of bike stuff is good- thermal baselayers, ace. Some festivals have now banned camelbaks which is a right bollocks, they’re perfect.
If it rains, waterproof footwear will make you feel like a king. I love my Official Festival Goretex Shoes.
Slightly more clothes than you think you’ll need- many more socks.
Very well wrapped weed- beat the sniffer dogs.
If you’re taking your beverages in, red wine in a box, dispose of box. But I used to just take one can, for tent pitching refreshment, then nip out and buy some. Works better some places than others. Same with food, basically the less crap you’re carrying, the better. Even if you’re driving it can be a route march to get into the camping area.