Kong Size S
I really like the Kong as it keeps your dog busy for a long time. You can fill the Kong with everything you want. You can be as creative as you would like to be. I usually fill it with e.g. mushed bananas, natural yoghurt, cheese, liver sausage pate, wetfood, etc. You can also mix things up or fill it in layers. Very often I am simply filling Rockys wetfood in the Kong as it seems to taste a whole lot better out of the Kong than out of his bowl. There are also specially designed treats that you can fill in the Kong which I however have not tried yet.
How does Rocky like the Kong? He loves it and gets very excited once I take it out.
Here you can find more information about the Kong.