I've hunted the drive in theater in my small town, and did really well on silver dimes. I think I totalled close to 20 in about 5 hunts. Funny that I never found a silver quarter. This drive in closed in the mid 80's. Of the 200+ coins I dug, most were 50's, 60's and 70's.
It's true that you're going to fight a ton of trash....but who cares? The good coins are there, you just need to pick through them. As has been mentioned, if it's too trashy, turn your disc up (I also have an Ace 250) to accept only dimes and quarters.
I found that the best spots were where the cars parked. There is very little ground debris at my drive in, and its possible from time to time to see portions of the speaker wires coming out of the ground.
MP