Blink and you miss them easter eggs are things I miss from classic comics.
Legion of Super-Heroes - The Great Darkness Saga
Thor Odinson and Karnilla The Norn Queen among the Daxamites who are under Darkseid's control.
Thor/Don Blake runs into Clark and Lois - Simonson's Run on Thor in the 80s
A tongue-in-cheek jab at DC in a Marvel What-If. Where Wonder Man (Simon) and Power Nab (Luke Cage) are gender bent as Power Girl and Wonder Woman. Note the "Distinguished Competition" on the poster.