Two eateries are both reasonably priced and casual.. The first is on Route 157, and is called the Black Bird Tavern. There are separated bar and restaurant areas.
However.
Lyman Orchards, for the win. On a sunny day, nothing beats eating freshly prepared food on the deck at Lyman's. The feel is very laid back and friendly, and the meal will fill you without emptying your wallet.
Rule of thumb in Middlefield: When in doubt, go with Lyman's.