Homer’s Odyssey.

Most modern adaptations present the stories Odysseus tells while visiting the Phaeacians as if they were the actual plot—but Homer’s audience would have known Odysseus as a notorious liar and trickster and wouldn’t necessarily have regarded his stories as true even within the context of the frame narrative. Homer’s epic focuses as much on the parallel stories of Telemachus and Penelope—I read the underlying story as their struggle to untangle Odysseus from his own web of deceptions and fantasies and bring him back to reality.

Just saw "The Play That Goes Wrong," and it is a two hour masterpiece of a play within a play.

I see videos of complete productions on youtube. Go watch one.

Threshold Kids in the Video game control it was a kids TV series to teach kids in the game about the FBC it was kinda bonkers

The “Busty Asian Beauties” running gag in supernatural. Was a magazine, website etc. Always gave me a chuckle.

Here's a website about those kinds of fake shows

https://nestflix.fun/

The hitchhikers guide to the Galaxy

In "day of the tentacle" Bernard could walk up to the computer and play the game preceding day of the tentacle. That was nice.

"The Deb of Night" radio show from Vampire: The Masquerade - Bloodlines is consistently hilarious and a great way to relax between the more horrific parts of the game. Bonus points for one of the regular callers guessing the plot of the game by complete chance and one of the main villains calling in to threaten whoever might be listening.

Sick Sad World from Daria. I'd love that show

"Ow, My Balls" is a claaaassic.

I'm a big fan of fake brands/products in anime. I don't know why, but they bring me joy.

Sweet Bro and Hella Jeff from Homestuck. Although it actually predates Homestuck and was retroactively converted into media-within-media, does that still count?

Crash Nebula and The Crimson Chin from Fairly Oddparents are both great

"S." by Doug Dorst is in itself a fake novel, where the "real" story takes place as handwritten notes in the margins to form the complete story, the fake novel itself just a prop. But reading the novel-within-a-novel-that-is-the-novel "Ship of Thesus" by itself is an interesting short surreal read.

Lestat being a rockstar and fronting his own band.

Seven on 7 is a series of TV news shorts and commercials from The Boys universe. It's a delightful little parody of Fox News. The whole series is available free on YouTube.

Welcome to Nightvale.

Invitation To Love the soap opera that a lot of residents of Twin Peaks, especially Nadine, seemingly adored.

Gotta love The Rise and Fall of Sanctuary Moon from The Murderbot Diaries.

McGarnigle from the Simpsons

Binky the Clown and The Buddy Bears from Garfield and Friends

Reptar from Rugrats

Watched Asteroid City recently for the first time. I thought the play within the movie (and the intertwining back and forth of storylines) was really interesting and entertaining.

"Address Unknown", a twilight zone/detective noir tv show within the universe of Max Payne.

Interdimensional TV on Rick and Morty

Tales of The Black Freighter in Watchmen. Comic in a comic.

The poems of Thomas Zane in the old gods of asgard concert in the book about max Payne in the video game Allen wake in the video game control in the amazing TV series the threshold kids in that song the janitor is singing.

... Pretty sure that's the continuity.