It was a given that The CW's big Arrowverse crossover Crisis On Infinite Earths was going to be brimming with Easter eggs to uncover. Thanks to the number of DC properties the episodes include, there are plenty of references and even more cameos to spot. Be sure to vote up the Crisis On Infinite Earths easter eggs you were most excited to see.
Burt Ward briefly reprises his role as Robin from the 1966 Batman TV series in the opening of Part One. He's seen walking Ace the Bat-Hound before reacting in a very Robin way to the red skies.
Lex Luthor becoming President of the United States is well-covered territory within the comics, but Smallville also gave hints that Michael Rosenbaum's Lex Luthor is elected President in the future. They even said he'd be elected in 2018, so when Tom Welling's Clark mentions him during Crisis, Lex is only a year into his first term.
Brandon Routh's Kingdom Come Superman reveals that Lois Lane, Jimmy Olson, Perry White, and countless other Daily Planet employees were killed years ago in an attack by "a reject from Gotham that felt like we didn't cover him enough. He played a practical joke." The attack was clearly pulled off by the Joker, and you can see the front page from the attacks in the scene where the two Supermen fight.
In Part 2, Lex escapes with the Book of Destiny to kill every Superman in the multiverse. One of the first Earths Clark, Lois, and Iris arrive on has already had their Superman killed by Lex and the tattered flag featured in the news report references the famous Death of Superman story from the comics in 1992.