I have lots of filling ideas!!

Scrambled egg and cheese
Peanut butter
Kibble
Natural yoghurt
Nature diet
Banana
carrot and other veg
meat eg.chicken,ham etc
Cheese
Kong fillings(the ones they make)
cat food(sometimes wet but usually dry)
treats

All are usually covered with a layer of peanut butter on the top(it covers the hole) and all can be frozen.

Oh and i mix all of them together,like i might have nature diet at the bottom then a layer of kibble,then some dry catfood and then a little nature diet again mixed with peanut butter to seal the top hole. And i usually stick a biscuit out the top too.

Cheese! Top favourite....

I've got a kong wobbler which is a biscuit dispenser. I put her food in it and it takes Millie about an hour to get all the biscuits out. I think i paid about £12 on ebay and it's worth every penny, i don't really use her normal kong often now.
http://www.kongcompany.com/products/for-dogs/wobbler/