Tuna Eyeball with Timothy DeLaGhetto and Ben Sinclair
Like its Marvel Cinematic Universe brethren, Avengers: Endgame is chock-full of references to past MCU films and the comic books. Not only did we go back in time to revisit Thor: The Dark World (who'd have guessed that would be the Thor movie we'd see again?), but long-promised thrill moments, like Cap wielding Mjolnir finally showed up.
There are also plenty of jokes, references, and other easter eggs hidden in smaller moments throughout Endgame, and Marvel fans will be curious about just how many they caught and how many they missed. This list will be updated upon rewatch after rewatch, but for now here's a list of the most prominent easter eggs in Avengers: Endgame -- presented in chronological order... which means real movie time, not quantum time.
Iron Man's callbacks
In Tony Stark's opening monologue to his helmet in space, he includes two references to previous Iron Man movies: "Don't post this on social media," which is a Stark line from Iron Man, and, "It's always you," which is what Tony said to Pepper when handing over Stark Industries in Iron Man 2.
Captain America's therapy
After the time jump, Captain America is working for a therapy group, just like Sam/The Falcon was when we met him in Winter Soldier.
A Russo brother cameo
Director Joe Russo shows up as a gay man in Captain America's support group.
Ken Jeong's book
When Scott Lang re-materializes in San Francisco, Ken Jeong (from Community and the Hangover movies) appears as a security guard, sporting a fake mustache. Not only that, but he's reading a collection of JG Ballard short stories called Terminal Beach, which includes the short-story "End-game."
Black Widow's ballet past
By Natasha's desk in Avengers headquarters, she keeps some worn ballet shoes, a reference to the ballet school cover she used for her assassin training in Age of Ultron.
Namor the Sub-Mariner
The underwater earthquakes off the coast of Africa referenced by Okoye in the holo-conference could be our first indication Namor the Sub-Mariner, a character whose rights are currently at Universal, is in-universe. Namor rules over Atlantis, the underwater city, and the Atlantians have found themselves at odds with Wakanda and their Black Panther several times in Marvel Comics history.
Endgame takes an image directly out of the Infinity Gauntlet comic series when it depicts Thanos' armor being used as a scarecrow.
Stones destroying Stones
Thanos destroyed the Infinity Stones with the Infinity Stones, which has only happened once in Marvel comics. When Captain America used the Gauntlet to stop an incursion of worlds during the Jonathan Hickman-written Infinity crossover event, the stones ceased to exist (until the entire comic book universe was relaunched with Secret Wars, and along with the relaunch, new Infinity Stones were introduced).
Iron Man's daughter
Tony Stark's daughter, Morgan, could be related to Morgan Stark, the comic book character who's Tony's eccentric and almost evil cousin. There seems to be no MCU origin behind her name.
A Hulk that is half-Banner, half-Hulk has appeared in the comics, where he's dubbed "Smart Hulk" (or "Nerd Hulk," as in the Ultimate comics). In the movie, Banner says he figured it out during months in the "Gamma Lab." At least, it sounds like "lab," but could be "land." In the comics, Gamma Land has occasionally referred to a block of the American Southwest where several Hulk/Gamma entities appear, though the MCU has so far made no actual mention of this.
New Asgard is in Tonsberg, the Norwegian city where Red Skull acquires the Tesseract in the beginning of Captain America: The First Avenger. Seems like Asgardians have had roots there for some time.
When Rocket and Hulk go to collect Thor, Korg is playing Fortnite using the Brite Bomber skin. The online Battle Royale-style game, where you can play looking like multiple characters (by using "skins," or different outfits) has partnered with Marvel to bring limited-time game modes to Fortnite for the Infinity War and Endgame releases.
Hawkeye's alter ego
Black Widow finds a murderous Clint Barton in his Ronin persona. Though he's never called that in the film, the Hawkeye-goes-rogue storyline and the costume in the film are both nods to the character's time as Ronin in the comics. The person Ronin kills on screen is a character named Akihiko, played by Hiroyuki Sanada. In the comics, Akihiko runs a group called the Shogun Reapers, who are kind of like a tech-forward Yakuza.
Captain America has been able to wield Mjolnir a few times in the Marvel Comics, going back to the first time in 1988's The Mighty Thor #390, but it wasn't until 2011's Fear Itself crossover event that Cap held it, yelled, "Avengers assemble," then used lightning powers, so let's peg Endgame's usage to that.
Captain America's shield
Captain America getting his shield broken by Thanos also happens in 1991's Infinity Gauntlet comic series, though that time Thanos was able to punch it into pieces instead of hacking away at it with a double-edged sword like he does in the film.
When Tony brings Captain America's shield back to him, Tony says, "He made it for you," referring to the Dominic Cooper version of Howard Stark from Captain America: The First Avenger. Tony has kept the shield since Cap abandoned it in Captain America: Civil War.
When Tony Stark and Ant-Man are hiding out at the top of Avengers tower, Loki says, "I'll take that drink now," and Tony Stark replies, "Enough posing." It's a reference to the iconic shot of all the Avengers looking down at Loki during the original end of that sequence.
Robert Redford shows up as Secretary Alexander Pierce (from Winter Soldier) and says the Tesseract has been in the possession of S.H.I.E.L.D. for almost 70 years. That means Nick Fury never told them about the time it was swallowed by a space cat.
Cap's alternate storyline
Steve enters an elevator with dirty S.H.I.E.L.D. agents in a sly homage to The Winter Soldier. In a comic book reference, Captain America says, "Hail Hydra." This could be in reference to the Secret Empire (aka Nazi-Cap) run of comics where Captain America was raised as a Hydra agent.
Seeing Peggy Carter is nice, but it's James D'arcy as Jarvis, driving Howard Stark in the past, that nods to the two seasons of the ABC series Agent Carter. D'arcy played Jarvis on both seasons of the show, but had not appeared as the character in any of the MCU movies. In universe, this Jarvis served with Peggy Carter before she took over S.H.I.E.L.D. and went on to inspire Tony Stark to name his first Iron Man Suit A.I. after him.
Stan Lee's requisite cameo
The late Stan Lee's cameo is as a younger version of himself yelling, "Make love, not war!" at a military base. The bumper sticker on the car in the movie reads, "Nuff Said," one of Stan's famous catchphrases.
When Howard Stark first stumbles across his son, he's looking for Arnim Zola, played by Toby Jones in The Winter Soldier and The First Avenger. We eventually learn that Zola is the villain responsible for S.H.I.E.L.D.'s having been taken over by Hydra.
When we visit Hank Pym's lab in the 1970s, he has a prototype Ant-Man helmet that matches the original comic-book design for the Hank Pym Ant-Man.
Iron Man's plaque
The "PROOF THAT TONY STARK HAS A HEART" plaque was a gift from Pepper to Tony in the first Iron Man.
A familiar kid
That teenage kid standing in front of Maria Hill at the funeral is Harley Keener (played by Ty Simpkins), otherwise known as "the kid from Iron Man 3."
When Steve Rogers says goodbye to Bucky before returning the stones, their lines about "doing something stupid" are pulled directly from their goodbye in The First Avenger.
The tan jacket
Old Man Rogers is wearing the same tan jacket Steve Rogers wore before getting the Super Soldier serum in The First Avenger.
A final Iron Man tribute
At the very end of the credits, you can hear the metallic pounding sounds of Tony forging the first Iron Man armor in a cave in Afghanistan.