I usually put some plain frozen non fat yogurt in it. First I'll put some kibble in, then seal it with the yogurt. But he tends to get bored with yogurt - little weirdo. So I more-so use canned food... just put the canned food in there with a spoon and freeze it and it makes for a nice little snack. Some use peanut butter but he's never liked PB - I tried it once a long time ago and just had a big mess to clean out because he wouldn't touch it