Defeat the Kraken Boss in World 3 (Coral Cave) to unlock Blaze The Cat.

Defeat the Titan Boss in Big Swell. Then, go to Seagull Beach on Windmill Island. Talk to the Koala with the guitar to hear the music, dialogue, and sound effects. Note: This will also unlock additional missions.

Collect all Sol and Chaos Emeralds to unlock Deep Core, the final extra Boss stage.