Sonic The Hedgehog Advance
Tails as partner
At the character selection screen, highlight Sonic. Next, highlight Tails and press Down. Then, highlight Knuckles and press L. Finally, highlight Amy and press R. Highlight Sonic and press A to select him. Tails will follow you during the game, but cannot be controlled.
Collect all seven Emeralds for all characters, then successfully complete the game with Sonic to unlock the Moon Zone.
Hidden sound test mode songs
To get more than the 39 default songs in sound test mode, successfully complete The Moon Zone, extra mode, or unlock the Super Sonic Ending. Three additional songs will be available.
Easy rings for Chao Garden
Play the first level. Press Start + Select + A + B to reset the game. Go to the Chao Garden and you will have the rings that were collected, but reset at the start of the next act of the level.
Rings for Tiny Chao Garden
Go to time trial mode and play any level you are good at. Get lots of rings, then go to the Garden. The game will add the rings to your total.
Infinite tries in Casino special stage
Get the checkpoint thing immediately before the spring. Then, try the special stage. If you fail, there is an extra life to the right, directly above the swinging boat thing. You can get unlimited tries because every time you die the extra life returns.
After collecting the Chaos Emeralds, complete the game with all the characters. Complete Final Zone with Sonic again and you will turn into Super Sonic and go to a new stage. Tap buttons to escape his traps. Use your Head Bash on him eight times to defeat him. Be careful, as you will lose rings as you fight.