Through the Rabbit Hole - S2-E1
Trivia: The boy's comment, "Everybody is kung fu fighting" at the ruined school dance, refers to Carl Douglas' song of the same name from 1974, two years before this episode takes place, following the fledging martial arts craze in America. This song has recently (in a slight variation) resurfaced in Kung Fu Panda.
Crisis on Infinite Earths: Part Two - S1-E9
Trivia: The version of Batman played by famous Bat-voice Kevin Conroy is a darker version of that from the graphic novel The Dark Knight Returns. The line "the world only makes sense when you force it to" is a version of a line from that comic (also used in Batman V Superman), also "Clark always said yes to anything with a badge or a flag." There are also elements from Batman Beyond, which first aired in 1999 (hence Earth-99), in which Conroy voiced an older Bruce Wayne mentoring a new Batman. The whole scene is full of nods to other versions - describing Kryptonite as "a little souvenir from the old hometown" is a Lex Luthor line from the original Superman movie, and him describing Superman as "strange visitor from another planet, with powers and abilities far beyond those of mortal men" comes from Superman serials from the 40s and 50s.
Trivia: Lars Mikkelsen, who plays Grand Admiral Thrawn and voiced him in Star Wars: Rebels, is the older brother of Mads Mikkelsen, who played Galen Erso in Rogue One: A Star Wars Story.
If This Be... M.O.D.O.K.! - S1-E1
Trivia: When M.O.D.O.K. and Austin are out on town, the places they visit make references to several other Marvel characters. The dragon that is serving them in the restaurant is Fin Fang Foom. The concert they are next shown to be attending is advertising Lila Cheney, who in the comics is a mutant who can teleport over interstellar distances. Lastly, the arcade belongs to Arcade, who is primarily an enemy of the X-Men. (00:10:27 - 00:10:58)
Trivia: The song playing on the radio when Rosebud breaks in, 'I'd Take a Bite of the Moon For You,' references the episodes 'Alone Together' and 'Tick Vs. Chairface Chippendale.' The song playing when El Seed breaks into the botanical gardens is a jazzy version of the show's theme song.