Among fruits, apples are one of the easy answers. The flesh is low in fat, high in water and fiber, and makes a satisfying crunchy reward — sliced fresh, or frozen for a warm day. Like any treat, it should stay a small extra on top of a complete diet rather than a meal in itself, since fruit also brings sugar.
The caveats are about the parts, not the fruit. Apple seeds, stems, and leaves contain cyanogenic glycosides — compounds that can release cyanide. A dog would need to chew a lot of seeds for a dangerous dose, so a swallowed seed or two is not cause for panic, but there is no reason to let seeds become a habit. The core is the more everyday problem: it is firm, awkwardly shaped, and can choke a dog or lodge in the gut. Core it, slice it, and apples are a treat you can feel good about.
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This page is informational and is not veterinary advice. If your pet has eaten something risky, contact your vet or an animal poison control line right away.