You ask for a spooky story if she has one, but in the end just let her decide whatever she wants.
"Okay! So: It was a spooky Nightmare Night like any other. Ponies dressed up and having fun, going door to door for sweets to give to Nightmare Moon-"
"Sweetie, try not to sound so happy if you're telling a scary story," Scootaloo interrupts with a grin.
"Oh. Right. Ahem!" Sweetie lowers her voice a notch, leaning toward the center of the group, "So there I was, collecting candy and dressed up as the sweetest little maid I could possibly be: Sweeping up candy with a little tiny broom I held with my mouth while Rarity talked with the adults handing out sweets. Everything was going great!"
"But suddenly, there were cries of terror from the bobbing for apples area! Ponies running everywhere, screaming in fright! I couldn't help but run toward it!"
Scootaloo slaps her own face with a hoof dramatically.
"You should get that looked at, Scootaloo," Sweetie says reflexively.
"Anyway; So I get to the big wooden tub they use for it, and all the apples are crawling around with spider legs coming out of them!"
"What? No way."
"Uh huh, she's right," Apple Bloom says, suddenly popping her head out from her sleeping bag, "I saw it myself! Apples with spider legs, just…crawlin' all over!"
"I think it was Nightmare Moon. Like I was saying: So, there I was, at the spider-apple stand, when one of 'em crawled on me and just started walking all over!"
Sweetie jumps up, dashing over to you to turn around and start rubbing her tail on the back of your neck. You spasm a bit in a fit of giggles before putting your hooves over your mouth to stifle the noise as Sweetie continues.
"They were all over me and just tickling me like crazy! I couldn't stop laughing and laughing. It felt like forever until finally Rarity came and saved me. She blasted them with her magic and they turned back to normal apples! Which were delicious!"
The room is quiet for a bit before Scoots asks the inevitable question.
"Is that it?"
"Yep!" Sweetie says, plopping back onto her haunches when she heads back to her sleeping bag.
Scootaloo sighs as Apple Bloom chimes in: "Still, it WAS pretty creepy. Nightmare Moon pulled a fast one on us while we weren't looking. Totally wasn't expectin' it."