I love going to the drive in! Its so fun and cheep! Plus you get to see 2 movies for the price of one and there not crap movies. We like to back our truck up, open the back, put all of the food and drinks out and then sit on lawn chairs. The kid(s) can stand up and stretch and have a great time.
There is a concision stand where you can buy food, candy and drink if you forgot stuff, but bring your own if you can. Also bring a little boom box for the radio station so you don't have to waste the battery in your car. We saw some people having to get jumped the last time we where there because they had their car radio on for like 4 hrs and it died.
Go give it a try its really fun and worth the $6 to get in!