A popular Halloween ghost/witch riddle is:
 
Q: What do ghosts put on their bagels?
A: Scream cheese.
 
“What do witches put on their bagels? Scream cheese” was printed in the Chicago (IL) Tribune on October 29, 1996.
 
“What do ghosts put on their mashed potatoes?”/“Grave-y” is a similar ghost riddle.
         
 
29 October 1996, Chicago (IL) Tribune, “Playing Around,” pg. 9:
What do witches put on their bagels?
Scream cheese.
 
Google Books
September 1997, Boys’ Life, ‘Think & Grin,” pg. 61, col. 1:
Ken: What do you get when you mix a bagel and a monster?
Ben: What? Ken: Scream cheese! — Ken Keeler, Temple, Calif.
 
29 October 1999, Anniston (AL) Star, “Humor,” pg. 5C, col. 4:
A ghost’s favorite snack is a bagel with scream cheese.
 
Google Books
Riddle Riot
By Lori Miller Fox
Illustrated by Sanford Hoffman
New York, NY: Sterling Publishing COmpany, Inc.
2003
Pg. 78:
What do witches spread on their bagels?
Scream cheese.
 
Google Groups: alt.humor.puns
Puns of the Weak 09-19-03 Part II The Rest of the Puns
Stan Kegel
9/19/03
(...)
What do ghosts put on their bagels?
Scream cheese (Margaret, 12)
   
Google Groups: alt.creative.writing
What does a ghost eat?
omjaram
10/21/04
(...)
Grains
Ghost Toasties      
Scream of Wheat
Pentagram Crackers with Poisonbury Jam      
Brain Muffins
                     
Entrees
Hungarian Ghoul Ash  
Frank ‘n’ Stein
Stake Sandwitch      
Halloweenie
Grave-y
Littleneck Clams      
Black Catfish
Warlocks and Bagels, with Scream Cheese      
                     
Side Dishes
Spook-ghetti          
Deviled Eggs
Artichokes            
Skullions
Scarrots                
Ghost Liver Pate
                     
Fruits
Adam’s Apples          
Nectarines
                     
Desserts    
I Scream
Boobury Pie              
Boo Meringue          
Ghoul Whip
Terrormisu            
Ladyfingers
Ghoulda Cheese        
Monster Cheese
                     
Beverages
Ghoul Ade
Coffin with Scream
Zombie
Apple Spider
 
Google Books
Spooky Sillies:
A Book of Ghost Jokes
By Mark Moore
Illustrated by Anne Haberstroh
Minneapolis, MN: Picture WIndow Books
2005
Pg. 19:
What do ghosts put on their bagels?
Scream cheese.
 
Google Books
Silly Jokes & Giggles
By Philip Yates & Matt Rissinger
Illustrated by Ed Shems
New York, NY: Sterling Publishing Company, Inc.
2005
Pg. 91:
What do ghosts like on their bagels?
Scream cheese.
 
Twitter
UCSDbookstore
‏@ucsdbookstore
What do ghosts eat on their bagels? Scream cheese. UCSD Halloween costume contest details http://on.fb.me/UCSDCostumeContest10.
4:56 PM - 7 Oct 2010
 
Twitter
Lauren Frost
‏@lauren_frost
“What does a witch put on her bagels? Scream cheese!” - Annabelle, age 6
7:22 PM - 19 Oct 2010
 
Google Books
Scooby-Doo Monster Jokes
By Michael Dahl
Illustrated by Scott Jeralds
North Mankato, MN: Stone Arch Books
2015
Pg. 14:
What do goblins like to put on their bagels?
Scream cheese!
 
Google Books
Even More Halloween Jokes For Kids
By Peter Crumpton
PeteyRF Creative
2015
Pg. ?:
What do witches put on their bagels?
Scream cheese!
 
Google Books
The Super, Epic, Mega Joke Book for Kids
By Whee Winn
Grand Rapids, MI: Zonderkidz
2016
Pg. ?:
What do monsters put on their bagels?
Scream cheese.