I love the IQ balls IQ Treat Ball Dog Toy - 1800PetSupplies.com
it keeps 2 of my dogs really occupied for a long time the other two just rather chew on a bone.
To switch it off I have used marrow bones that once they clean out the marrow I will refreeze with wet dog food, ground beef or chicken broth with pieces of chicken in there. They love it and wont stop till they are done. By that time they need a nap.