Can I bring a backpack with seat cushions, jackets, water etc?

Inna is exactly correct

I posted a review w the specific or check online they have all the rules on there

You can't bring backpack, they have new restrictions about the size of the bag you can bring. But you can bring a jacket and a seat cushion, just not in the backpack. No outside drinks allowed, but I see a lot of people are picnicking next to the parking lots, great views there, before entering the concert grounds.