My parents took us to the drive-in one time each summer. I think it took my mom a year to stop twitching from the experience.
Like others, we went in our pajamas on the theory that we'd be put in our beds directly on returning home. Nice idea; too bad it didn't work.
There was a rickety old playground in front of the screen. As soon as we saw the other kids playing up there in their pj's, the cries arose to go play, too. Eventually, my parents would give in and we'd return to the car, sweaty, filthy and occasionally bleeding (I did mention the equipment was old and rickety, didn't I?)
There were also trips to the bathroom. Nobody could ever find their shoes without having to turn on the overhead light.
No treats from the concession stand. We always had a jug of Kool-Aid and a bag of home-popped popcorn, both of which ended up on the floor and seats of the car and the afore-mentioned pajamas.
There would be constant jockeying for the best positions and much whining about how someone was taking up too much room, getting in the way, singing along with the movie, etc, etc.
Then, when we got home pajamas would have to be changed and dirty hands, feet, faces would have to be washed.
My mother would swear "NEVER AGAIN!!!!!" but we'd go again next summer.