Kongs

yes and that is why kongs are made out of such thick rubber and so are virtually indestructable...
(hence getting the right size is so important, my parents cocker destroyed vixies tiny kong...while his is still fine...*pout*)

the spaniel likes peanut butter but this is difficult to clean and youll need one of the preassure hoses on your sink to blast out any missed peanut butter (the same with the stuffing the company makes for the purpose its like cheese in a squirty can)
vixie likes hers used liek a puzzle cube with some dry food or treats broken small and put inside which then fall out as she tosses the kong about...both also like having a cookie wedged in there for them to try and chew out.

Fizzy just loves his kong and will play with it for hours , I don't put anything in it he loves it just as it is

what size/colour kong did you get ?

They have special made things to put in kongs at petstores. Unfortunately, Seiah isn't to interested in his kong he prefers his toy carrot.

try ice cubes, they always slip around on the ground otherwise

I take Cooper's food and put it in there. I will take a little treat and mix it in so the food doesn't get boring after a while. I've used bacon, ham, cheese, green beans and plain dog treats. Then I take a small dab of peanut butter and "cap off" the opening. Sometimes, I'll put some peanut butter on the inside as well - just wipe it along the interior wall with my finger.

I've also put small toys inside, flavored ice cubes (I made some with watered-down beef and chicken broth), green beans (Cooper's favorite human treat and they're good for your dog), and just left it empty as a play toy. Be sure to put something really good at the very bottom so it gives your pup a reason to get everything out!!

iv seen the makers of kong make a tube of paste like stuff that fills them in different flavours iv never used it but they look good