The sky's the limit. Anything that you can clear out in time for the evening show can be put in there during the day, unless you have the land to run all of it at once.
The busiest place I've seen has a putt-putt course, an ice-cream parlor, pizza business, walk-in restaurant and outdoor concert stage, all surrounding a twin drive-in.
A couple of my friends have drive-ins with carnival-style rides
Here in Washington, we have a drive-in that has an attached arcade building and a seasonal go-kart operation. They're able to operate year-round in a climate where the rest of us close for the Winter.
Flea markets are an obvious choice, though it does take a sturdy constitution to run one... the patrons tend to be a bit different from your drive-in crowd.
Also consider what extra activities will do to your workday. It takes much of the day to prep a drive-in business for a full night. You're basically shifting the business day... and not necessarily working a shorter one. Adding daytime activities will make you old fast, unless you're planning to have some of it managed for you. Many of us find it's best to concentrate on making the drive-in business the best we can provide in our area.
It can be a great business to be in... good luck!
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