My dog likes apples. I give her about a 1/4 apple, cut into tiny pieces as treats. At apple time, she often picks her own from the low branches of the trees in the orchard! So I make sure she doesn't get any apple seeds too because they are toxic.
Grapes are definitely toxic for dogs. NO grapes! Raisins are the same, ditto sultanas, currants, etc. I've heard of dogs who like bananas, but mine doesn't.
Corn....