Apples are fine, but not the stem or pips. You can also grate it over their dinner and it sometimes encourages a reluctant eater
Be careful with oranges - and other citrus fruits, they are too acidic for dogs.
Also NEVER feed grapes - some dogs seem to be fine with them, but there have been quite a few instances of dogs being poisoned and sadly dying from eating grapes {and raisins/sultanas}