One of the main characters in "Dreamcatcher"note The one about girls at summer camp trying to stop a spider creature that feeds off nightmares is played by Michelle Creber (which any brony worth his salt would know is the voice of Apple Bloom on both My Little Pony: Friendship Is Magic and Equestria Girls).
Another My Little Pony: Friendship is Magic voice actress on an episode of The Haunting Hour: Alice's mother in "My Old House" voices Princess Celestia.
Gorgonzola (Will Shadley) plays Aaron in "Spaceman".
Jeff (the square-headed kid) is Sean (Sean Giambronel) in the season four premiere episode "I'm Not Martin". It's not hard to figure out (unless you've never seen Clarence), since Sean sounds like Jeff when he yells, "I'm not Martin!" (which is often in this episode).
Talking to Himself: As mentioned above, Tom Kenny not only plays Uncle Howee on the episode of the same name, but is also the voice of the rabbit puppet, Loomis, and they interact with each other throughout the episode.