Check out the full journey to unlocking the post-game rewards by collecting all 120 stars:

While the game does not have a formal "post-game" save state (it resets to the castle after the final Bowser battle), reaching 100% completion unlocks specific rewards:

: Once you collect all 120 Power Stars , the cannon in the castle courtyard opens. Use it to blast to the roof to meet Yoshi , who gives you 100 lives and an upgraded triple jump that leaves a trail of sparkles.

: Running around the post on the small island platform causes it to release coins.

: You can return to Cool, Cool Mountain for a harder rematch against the Big Penguin in the "Big Penguin Race" mission.

: In the desert, circling certain pillars can trigger specific events or coin spawns. Post-Game Content

: Running around the post that the Chain Chomp is tethered to three times will set it free, allowing it to break the bars behind it and reveal a Power Star.