Good Luck on your venture. Yes, your food sales are very important...
Here's my take on it. Go with the staples: Popcorn,Soda,Nachos,Super Pretzels,Candy,slush puppie, Cotton Candy,Sno-Cones,Pizza,Hot Dogs.
Now, you can go a step further by getting a grill and fryer... Burgers and Fries would be the next step. As you stated, some places had 10 different fried items. Since you have the fryer you'll just have to try and see what sells, maybe cheese curds will sell great but onion rings won't... who knows. Doesn't hurt to try though. Also remember to consider food cost. Customers don't want to pay $4.00 for cheese curds but you may have to charge that because they are an expensive cost item. But onion rings aren't expensive so you can sell them cheaper etc... Do specials with your great items so your place becomes known for them. Wouldn't it be great to hear "lets get a foot long hot dog at the drive-in, they're awesome"...
You can also add menu items by just doing little things. You can have hot dog on your menu, which leads to a cheese dog (using your cheese for nachos) Chili-cheese dog, onion dog, jalapeno-pepper dog, chicago dog, etc. Be creative.
Another thing you could do is this. Rather than have all these items on you menu, you could have a "sandwich of the week" or something. That way you could have your "meatball sub" or whatever . Your cusomers would get used to looking for your "Sandwich of the week" and may try different items. That way if something does do really well, you could decide if you wanted to carry it all the time. You could also check with your foodservice provider, as they have monthly sales a lot. You could get shaved ham on special from them then make one of your sandwiches of the week with it, and make extra profit because it was on sale.
Don't forget the NON-FOOD items you can sell. Bug spray, Glow in the dark necklaces,rings,bracelets etc...
Check out
www.gotglo.com they have good prices
Good Luck
Jeff