Cousin Collecting level:
Find all of the cousins, then talk with the boy in the yellow shirt.
Million Roses level:
Complete the Cousin Collecting level, then talk with the rose.
Complete the Cousin Collecting level for a second bonus movie.
Complete the game to access the Memorial option. This allows you to view movies and listen to music from the game.
Staff Credits level:
Complete the game. Talk with the girl in the yellow shirt. You can now play the level that featured the game's credits.
Complete all levels with a good enough time to unlock all the comets. Fly into the sky from the Hub screen to reach the Cosmos Screen. Every comet is assigned a button. Press the buttons to view all the comets.
Thanks to Revolution readers BillyBlankz and WillBilliamson!