My dogs are sweet potato fanatics.
What we do is boil up a big pot of sweet potatoes, mash them, and then portion them out into ice cube tray and freeze. When we're in a hurry and don't have a lot of time to stuff kongs, we grab a potato cube and shove it in along with a treat or two. Instant doggie fun.
We actually discovered this by accident. We have a parrot who eats warm sweet potatoes before bed every night, so we'd freeze up trays of potatos for him. It just happened that the dogs are nuts about them too.
Works with butternut and acorn squash too.