Not super familiar with drive-ins. You'd probably want an old flashlight (you could maybe use that to illuminate any beasties inside) a roll of tickets, a hole punch for punching 'em. Maybe you could download a few coming-soon ads for movies off the web. The photos I'm looking at also have lots of signage - pricing, movie title and lots of rules... no reentry, no alcohol, no dogs, turn off your headlights etc. Maybe a help wanted sign? Probably a magazine or book for the person working there to read. If you're doing a 1950s Drive-in you could go retro, maybe an old monster-movie magazine. Some half eaten food from the concession. If you going for an older drive-in, you could probably mockup an old speaker like they used to hang on the cars back in the day. I'm sure those things broke all the time and likely sat around waiting to be replaced. Maybe some lost car keys?
Are you gonna play the "Let's all go to the lobby" short? That never gets old. I think I'd try to come up with a little backstory and base whatever props off of that, taking into consideration who was working and what a typical day for them would be like. It's gotta get kinda boring in those little booths.