Anything you can think of really, a few dog biscuits, left over pate, cheese, pilchards, sardines, ham, chicken, vegetables, held together with a small spoon of peanut butter and a hint of marmite!
Or if you like it fiendishly difficult, block up the small end with a piece of bread, break in a couple of eggs and a few biscuits and cook in the microwave till the eggs are hard! How long do you think it could take to lick an egg out of the middle? Or how about a few dog biscuits layered with some grated cheese - melted in the microwave then chilled in the fridge till it is hard! Or better still serve it frozen straight from the freezer! That takes ages to eat and dogs really enjoy the experience.
Dogs experience real enjoyment at taking time over their food, isn't it a shame that most people feed their dogs from a bowl and it is all over in 20 seconds! As long as the food is part of their recommended daily amount, then why not spread their food throughout the day with a Kong. A very common reason for aggression or strange behaviour is due to low blood sugar or hypoglycaemia. It is a bad mistake just to feed your dog just once in the day - they should have several smaller meals throughout the day, levelling out blood sugar. Kongs are brilliant! How do you eat yours?