oh wow the options are endless! Peanutbutter, they have stuff you can also buy at the pet store in like a cheez whiz can you can put in there! I have taken milk bones and shove them in there also. You can pretty much put anything in there!
I never thought a dog would like Peanut Butter.. I mean REALLY.. Dogs don't farm.. but boy do that love it. I stuff with that and then put unflavored yogurt in the center and freeze the thing and give it to her.
Can keep her busy for about 20 minutes (determined dog).