Twist N' Treat
If you only ever buy your dog one "work-to-eat" toy, this should be the one. The hard-rubber Twist N' Treat -- part of the Busy Buddy line of toys -- can be used with treats, dry food or wet food. Twist it open so there's lots of space for yummies to fall out, and it'll be easy enough for even a tiny puppy; twist it tight and it becomes much more of a challenge, as your dog tries to maneuver out the goods. For an extra challenge, fill it with baby food, sweet potato or peanut butter, and then freeze the whole thing. So many options!