Best rule of thumb is to think in terms of the wild, and what they'd eat there. Chocolate buttons, say, would be short in supply
Our second dog used to love peeling tomatoes. Never ate them - just peeled them. You've got wonder what he got out of it. As a pup he consumed a fair bit of carpet and bedding. Best not to discuss what came out the other end, but it's a wonder he didn't get a blockage. Strange things are pets. That said, the hamster used to spend ages stuffing, unstuffing and restuffing his cheek pouches (incredible things). When I'm re-arranging the freezer to get everything in, I sometimes think that maybe he wasn't all that daft.