Sonic Advance features a few unlockables and subtle easter eggs for dedicated players:
- Sound Test: Complete the game with all seven Chaos Emeralds to unlock the Sound Test option on the main menu. This allows you to listen to all the game's music and sound effects.
- Tiny Chao Garden Connection: As mentioned, connecting your GBA to a GameCube with Sonic Adventure or Sonic Adventure 2 Battle allows for Chao transfer. This isn't an in-game unlock but a significant feature.
- Character-Specific Endings: Each character has a unique ending sequence if you complete the game with them. If you collect all Chaos Emeralds, you'll get the true ending regardless of character, but the character-specific epilogues are still worth seeing.
- Hidden Messages: While not widely documented, some older Sonic games on GBA had very subtle graphical easter eggs or developer messages. Keep an eye out for anything unusual in the background art.
- High Score Tables: While not an 'unlockable,' striving for the top spot on the high score tables for each act and overall is a form of achievement and replayability.
The primary unlockable is the Sound Test, which is a nice reward for full completion. The true ending with all Chaos Emeralds is the most significant 'unlock' in terms of story progression.