I used to live right around the corner from Cascade for a while. I love the drive-in itself, but there ain't much special around there, as has been noted.
Scoobys is home of the worst chili dog I've ever had, but they do have Green River soda on the fountain and there's something to be said for that. However, if you get Scooby's make sure to hide it well, because there are signs posted at the entrance which say all Scoobys food will be taken from you.
There's something of a feud between the owners of the Cascade and Scoobys because the neon at the corner of Prince Crossing and Rt. 64 is very bright and the Cascade thinks they should turn it off during the movies so as not to affect the picture on the screen. Scooby's disagrees and the light remains on, even past the stand's closing time.
Augies sells a pretty good sub for a decent price and their italian beef is good as well. There's also a Steak & Shake and an Arby's in the same plaza, for what it's worth. Just north on Rt 59 is Din Hua Chinese if you're hankering for that--it's okay, nothing special.
My preference would be to make a run out to St. Charles further down Rt. 64 and get some wings from Frantastic Wings or even a pizza from the Geno's East outlet, both in the Tin Cup Pass shopping plaza at 1534-1590 E. Main Street (Rt. 64). Or for mexican food, I've never gone wrong at La Villita on Roosevelt Road, about 15 minutes south on Rt. 59 and a short jog east onto Roosevelt.
Side-note: I haven't been to Cascade for about a year and a half, but IIRC the food in the concession stand goes on sale for a dollar halfway through the second feature, and it's worth almost every penny at that point.
Enjoy the show.