A riddle involving Eggo brand frozen waffles is:
 
Q: What do you call a waffle on a California beach?
A: A Sandy Eggo.
 
The “Sandy Eggo”/“San Diego” pun has been cited in print since at least 1987.
 
     
Wikipedia: San Diego
San Diego (/ˌsæn diˈeɪɡoʊ/; Spanish for ‘Saint Didacus’; Spanish: [san ˈdjeɣo]) is a major city in California, United States. It is in San Diego County, on the coast of the Pacific Ocean in Southern California, approximately 120 miles (190 km) south of Los Angeles and immediately adjacent to the border with Mexico.
 
Wikipedia: Eggo
Eggo is a brand of frozen waffles in the United States, Canada and Mexico, which is owned by the Kellogg Company.
 
13 December 1987, Chicago (IL) Sun-Times, “San Diego: From apple pie to pandas at the zoo” by Howard Fensgtemaker, RV Odyssey, pg. 4:
I dropped a waffle on the beach and exclaimed, “Look at this sandy Eggo.”
       
Google Groups: rec.humor
Canonical List of Punny Book Title
M N Nelson
5/20/93
(...)
407. Southern California Waffles: Sandy Eggo*
     
Google Books
Nutty, Good, Clean Jokes for Kids
By Bob Phillips
Eugene, OR: Harvest House Publishers
1995
Pg. 43:
Ryan: What city are you in when you drop your waffle in the sand?
Reginald: Who knows?
Ryan: Sandy Eggo.
 
16 July 1996, Ukiah (CA) Daily Journal, “Jokes and Riddles,” pg. 12, col. 3:
What California city is named for a frozen waffle dropped on the beach?
Answer: Sandy Eggo.
Kaleena “Kali” Quarles,
Age 9 3/4
 
16 February 1998, Los Angeles (CA) Times, “Laugh Lines: Pint-Sized Punch Lines,” pg. E4, col. 3:
What do you call a waffle that falls in the sand? “Sandy Eggo.” (Katie O’Rourke, 9, Solvang, Solvang Elementary)
 
Google Groups: alt.humor.puns
Puns of the Weak 03/15/02
Stan Kegel
3/16/02
(...)
What California city sounds like what you’d have if you ate a certain brand of waffle at the beach?
Sandy Eggo (Clynch Varnadore)
 
Reddit—Jokes
What do you call an international criminal waffle that you dropped at the beach?
submitted April 25, 2012 by andrewp123
Carmen Sandy Eggo
 
Reddit—Punny
Where do you find waffles that like beaches? (self.Punny)
submitted May 9, 2012 by asleeponasunbeam
Sandy-Eggo!
     
Reddit—San Diego
What do you call a waffle at the beach? (self.sandiego)
submitted July 18, 2012 by emqz
A sandy eggo
 
Reddit—Dad Jokes
Not a dad, but I made this up and thought you might appreciate it. (self.dadjokes)
submitted August 27, 2013 by andrewp123
What do you call a criminal waffle that you dropped at the beach?
Carmen Sandy Eggo
 
Reddit—Puns
If you drop your waffle on the beach in San Diego… (self.puns)
submitted November 10, 2014 by armyjackson[🍰]
You have a sandy eggo in San Diego.
 
Reddit—Dad Jokes
What do you call a Waffle on a California beach? (self.dadjokes)
submitted January 15, 2015 by youtz1
A sandy-eggo.
 
Reddit—Cleanjokes
What do you call a waffle on a California beach? (self.cleanjokes)
submitted June 13, 2018 by michaelbaysucks96
A Sandy Eggo.