New Game+
To unlock the New Game+ feature, defeat Lavos at the end of the story with any ending. With New Game+, you can re-experience the plot of the game with your current character, retaining his level, his magic abilities and his gear. When you start a New Game+, only your money doesn't carry over.
Endings
You can experience "Chrono Trigger's" 13 different endings by defeating the end boss Lavos at varying points throughout the story. For "Beyond Time," revive Crono at Death Peak, and then kill Lavos, and for "Reunion," kill Lavos while Crono is still dead. For "The Dream Project," kill Lavos at the start of New Game+ or at the Ocean Palace. Save Leene and Marle and defeat Lavos before the End of Time for "The Successor of Guardia," and defeat him after the End of Time but before the Middle Ages for the "Good Night" ending. Obtain the Hero's Badge in the Middle Ages for "The Legendary Hero" ending.
More Endings
Obtain the Hero's Badge in the Middle Ages and defeat Lavos before the Gate Key is stolen for "The Unknown Past" ending. After obtaining the Gate Key and before you give the Masamune weapon to Frog, defeat Lavos for the "People of the Times" ending. For "The Oath," defeat him after Frog has Masamune and before you fight Magus, and for "Dino Age," defeat him after you fight Magus but before you fight Azala. Defeating Lavos after Azala and before Schala opens the door gets you "What the Prophet Seeks," and defeating him after Schala opens the door gets you "Memory Lane." The final ending, "Dream's Epilogue," occurs when you defeat the Dream Devourer beyond Time's Eclipse.
Lost Sanctum
The Lost Sanctum is a dungeon only present on the Nintendo DS version of "Chrono Trigger." To access it, unlock the Black Omen in the year 12,000 B.C, at which point entrances to the Lost Sanctum will appear in 600 A.D. near Ozzie's fort and in 65,000,000 B.C. near the Sun Shrine.