I have given mine escarole, collard greens, turnip greens, apples, watermelon, grapes, bananas, dandelion greens (I've heard they can have the flowers too), green beans, carrots, mustard greens, various lettuces. He's a piggy and not too picky
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I hear things like Kale, spinach, carrot tops, celery stalks, cauliflower and broccoli should only be given occasionally because they're higher in some things than others.
I thought that this page seemed to have some pretty good information.
Food Information Chart