Okay, so I thought the Flatwoods Monster in GF would make a great episode. Here's a sort of... transcript I came up with for the episode.
Not-So-Little Green Man[]
Ext. Mystery Shack. Late at night.
Int. Mystery Shack.
Stan sits on a chair in the gift shop counting money.
Stan: KIDS! I need you!
Dipper runs down the stairs. Mabel trips and falls down them to the bottom.
Dipper: Yeah, Grunkle Stan?
Stan: You and your sister need to go hang up some more signs for the Mystery Shack.
Mabel: Woohoo! Forest hike! Let's go, Dipper!
Dipper: Hey, wait a minute I-
Mabel grabs Dipper by the arm, grabs the signs, some nails, and two hammers, and runs off with Dipper in tow. Literally.
INT. Forest. Night.
Dipper: I can't believe you would just drag me into here like that, Mabel!
Mabel: Ah, lighten up, goofus!
Dipper hammers a sign into a tree.
Dipper: Yeah, but I was gonna tell Grunkle Stan that I had some... things to do.
Mabel: You were gonna write in that nerdy book thing with the hand on it about how much you wuv Wendy?
Dipper: Uh... maybe. But that's not for you to-
Just then, a huge ball of light passes over the twins and lands somewhere nearby.
Mabel: Awsome! The moon crashed! Now I have a reason to practice my moon walk!
Dipper: Uh, Mabel, first off, the moon's still in the sky, second off, the moon-
Mabel runs off without listening to a word Dipper says. Dipper sighs and runs off after her.
OTHER FOREST AREA.
Mabel stares awed by the huge ball of light on the ground.
Dipper: Mabel, you can't just-
Dipper notices the light.
Mabel: Who needs flashlights? We have this thing to light the whole forest!
A loud, sharp hiss emanates from a nearby tree. Dipper notices two yellow lights hovering by the tree. He shines a flashlight on the two lights. A 12 foot tall lizard-like monster in a hoversuit with glowing yellow eyes is revealed.
???: HSSSSSSSSSsssssss......
Dipper & Mabel scream and run off, with the creature in hot pursuit. Or what counts as hot pursuit when you hover at up to 2 and a half MPH.
INT. Mystery Shack. 3 minutes later.
Stan walks into the room carrying a box labeled: "fragile plates" and "not really cursed".
Dipper and Mabel bust into the room, and bump into Stan, who drops the box. A loud smashing of porcelain is then heard.
Stan: NOOO!! Not my China! I was gonna open a new exhibit! Ancient cursed plates!
Dipper: No offense to the plates, Grunkle Stan, but I think we have something more important to deal with right now!
Two yellow lights can be seen moving towards the window. Stan is oblivious.
Mabel: Stan! Look at the window!
Stan turns to the window.
Stan: Oh, that's just car headlights!
A green mist sprays from under the yellow lights, and smothers the area. A chicken walks into the mist and coughs. It soon falls over, and gets back up and runs off, it's health probably in a bad state.
Dipper: Is THAT a car?!
Stan: I know a guy who has a car that could do that.
A two-fingered, green scaly hand touches the window, burning the glass, luckily not all the way through.
Dipper: How 'bout now?
Stan turns to the window again, sees the monster, and screams. He pulls out "The Most Distracting Object in the World", and pulls the string. The monster stares at the spinning spiral, mesmerized.
TRANSCRIPT END[]
And, that's all I've come up with so far, though. I'm still working on the rest.