I've got two type of Kong - an traditional easy one for Daisy who would otherwise give up if she had difficulty getting the food out, and a more difficult kong ball for clever old Jazz who has learnt that to get the food out she must pick it up in her mouth, lean her head backwards, and let the food drop into her mouth - always amazes me to watch
I tend to cut sausages up and put them in the kongs as it never fails to keep them occupied until the last bit has gone