Clair Obscur: Expedition 33 has no traditional post-game content, but there’s still plenty to explore after completing the game. Once you reach Act 3 and Esquie gains flight, you can return to Lumiere or delve into the fully-opened world.
Maximizing your endgame experience involves several key activities:
Relationship Levels: Focus on maxing out relationships with party members to unlock optional dungeons and boss fights that provide insight into the game’s story.
Optional Bosses: Defeat these bosses to test your skills, which can be found throughout the Continent. Each has unique challenges and rewards.
Endless Tower: This tower offers 11 stages of combat encounters that require tactics and timing. Completing it earns you the “Endless” trophy/achievement.
Clean Up Collectibles: Explore the world for hidden goodies like Expedition Journal Locations and All Weapons, now accessible with Esquie’s flight.
New Game+: Start a new game with your save wiped, offering a fresh experience with increased character strength and retained skills. This option allows you to replay the game, explore different endings, or challenge yourself with tougher enemies.
Source: https://www.ign.com/wikis/clair-obscur-expedition-33/Postgame