9. Destroy Lavos In The Prologue - Chrono Trigger
The secret developer ending in Chrono Trigger is one of the earliest examples of that trope in video games. Although it's played off as a joke, you can reach the ending by fulfilling the main objective of the game within the first hour, without having to indulge in any exposition, character arcs, or plot developments.
Lavos, the world-ending parasite that serves as the main antagonist of the game, is encountered by the player in the first stage. Being the final boss of the game, you're obviously not supposed to win, but it isn't impossible. By using exploits such as power-grinding or simply using a cheat device, you can stand a chance against Lavos and defeat him at the Ocean Palace, without having to beat the game or use the New Game Plus.
Defeating Lavos early awards you with the secret developer ending, in which your character is transported to a literal developer room, seeing Chrono Trigger's various big name creators congratulate you.
It's a pretty ironic reward, considering that you just ignored most of the stuff they worked so hard on, and skipped right to the end.