Soak kibble in low sodium broth, mix with sardines and freeze. This one kept my two Lurchers quiet for ages. If you smooth over the top of the large hole it takes them a while to get through there before they even get into the kong itself.
Lots of people use loose dry kibble and peanut butter for it to stick to - but peanut butter had an, erm, interesting effect on my dogs’ wind production, iyswim. If I wasn’t freezing and didn’t need it to last a long time I would use either wet dog food or mushed sardines to hold the kibble instead. (If you do decide to try peanut butter make sure it doesn’t contain xylitol - sometimes listed as wood sugar, birch sugar, and birch bark extract.)
Greek yoghurt, blueberries and banana, again, frozen was popular with mine as well.
Basically, if you want it to last a while, you need something they like to eat, plus something to act as the glue. (Eg Scrambled egg, chicken, sweet potato, pumpkin, mashed potato, cheese)