I use the Jumpin Jack Kong. I put either peanut butter in the grooves or squish regular american cheese slice pieces into the grooves. All of mine love it!! They don't like any other rubber toys, but go crazy over this if there is cheese or peanut butter in it. The grooves really help clean their teeth too! I ended buying enough of them so they each had their own since they were fighting over them. Once the food is gone they lose interest. But it does take quite awhile and they have to use their teeth to get it out. You can hear their teeth being rubbed by the rubber.
Here is a picture:
jumping-jack-kong-toy.jpg