Didn't think they allowed sharp object, like the multi-tool.
Not so keen on the inflatable tent. I usually have to blow up the air bed a few times during a festival. Not just because of my weight on it overnight as it will go down a bit between a morning blow up and night time. Would not like the idea of waking up to the tent sagging down and touching my nose.
My essentials include a freezer bag (one beer out, one beer in). Instead of chill packs, freeze some bottled water (cheep supermarket mini bottles) as you don't have to carry your chill packs home. Instead you can drink them before that drive home.
And don't forget gaffer tape. Can fix lots of problems. Some idiot slashed your tent with a multi tool? Tape the hole shut to keep the weather out.
Bin bags inside the rucksack to keep your clothes dry is essential. Even if it isn't raining. those cans of fruit cider in your bag can still pop and leave you swimming in booze, and not in a good way. They can also be used to clear the rubbish out your tent at the end.
Some festivals let you pre-order booze. Watch the prices though, as you may pay more to collect. Comparing it to the cost on your muscles for dragging it in with everything else in one trip (especially if you travel by bus) or making that second hike to the car.
I was at one that did an exchange program. Hand over any unopened can of beer and get one of the sponsor's tins, freshly chilled. It explained why someone was carting a case of "value" lager in to the camp site.
And if there is a squad of you, a large tent, rather than separate tents, might be handy. Gives you larger dry area outside your bed to put boots and coats. And, if its raining, you can sit in your festival chairs in the dry drinking your warm lager.