Mine would eat the cardboard, haha, but he does enjoy when I put enrichment treats together. We have a little, hollow rubber "iceberg" for freezing, so I'll fill it with things like pumpkin, pepper, strawberries, apples, whatever's on the menu and then give it to him. He quite enjoys it, though last time he took some chunks out of the rubber, whoops. He's a "maximum destruction chewer", so I've got to be on top of taking certain toys out of his reach once we are done with their intended purpose.
He also has a rubber pinecone. It's got the little petal things on the outside, and the inside has a rudimentary "trickle dispenser" system of small shelves so that treats can come out slowly. I've smeared pumpkin and banana on the outside, sometimes I freeze it, then I put either broken up treats or kibble on the inside. He doesn't like eating food out of the dirt on the ground, so it's a great toy for when he has to be in the crate for longer (due to rain, heat, whatever).
The only problem I have with these toys is that they are "too small".
Good enough size that he enjoys them and they don't pose a risk, but not big enough in the food compartments. He gets about 4.5 cups of food a day, and the pinecone doesn't quite even fit half a cup. I have had my eye out for more treat-based enrichment toys whenever we go to the store. A lick mat would be awesome, though it'd have to be thick, solid rubber, not silicone. 
