Early Episodes (Season 1-2):
* "The Enchiridion!" (Season 1, Episode 1): Simon first appears in a flashback as a human scientist who finds the Enchiridion.
* "The Witch's Garden" (Season 1, Episode 12): Simon is seen in another flashback, where he is being seduced by the Ice King's crown.
* "The Jiggler" (Season 1, Episode 16): Simon appears in a flashback, fighting off the Lich.
* "Ocean of Fear" (Season 1, Episode 26): Simon is seen in another flashback, where he's trying to protect Marceline from the Lich.
* "Evicted" (Season 2, Episode 2): We see Simon in flashbacks trying to hold onto his sanity as the crown's influence grows.
* "The Real You" (Season 2, Episode 16): Simon is seen in another flashback, where he and Betty are together.
Later Episodes (Season 3 and beyond):
* "Memory of a Memory" (Season 3, Episode 14): This episode is primarily focused on Simon and Betty's relationship and the start of Simon's transformation.
* "I Remember You" (Season 3, Episode 15): This episode features a flashback to Simon and Betty's wedding day.
* "Holly Jolly Secrets" (Season 5, Episode 2): Simon appears in a flashback, where he tries to find a cure for his condition.
* "The Tower" (Season 5, Episode 9): Simon appears in a flashback, where he is struggling with his memories and the crown's influence.
* "Frost & Fire" (Season 6, Episode 12): Simon appears in a flashback, where he is being haunted by the Lich.
* "The Mountain" (Season 7, Episode 21): This episode features a flashback to Simon's attempt to destroy the Lich's remains.
* "Elements" (Season 8, Episode 4): Simon appears in a flashback, where he is trying to protect Finn and Jake from the Lich.
Notable mentions:
* "Simon & Marcy" (Season 4, Episode 2): While Simon doesn't appear physically, this episode is all about Simon and Marcy's past relationship.
* "The Thin Ice" (Season 7, Episode 2): This episode features a brief glimpse of Simon's human form.
These are just some of the most prominent episodes featuring Simon. He appears in many others as well, often in flashbacks or as the Ice King's past self. You can search for episodes featuring Simon or specific events related to him on the Adventure Time Wiki or streaming platforms like HBO Max.