Sonic Adventure Cheats

Play Emerald Course in chao race
Beat the rest of the four courses. Note: Don't delete your your chao data or else you will have to do them again.

Unlock characters
Amy: Talk to her in the Casino as Sonic after finishing Sky Chase.
Big: Defeat Chaos 6 as Sonic.
E-102 Gamma: Defeat Gamma after finishing Sky Deck as Sonic or Tails. An alternate way to unlock him is to finish Twinkle Park as Amy and watch the FMAs after it.
Knuckles: Defeat him after finishing Ice cap as Sonic or Tails.
Miles Tails Prower: Finish the C mission of Emerald Coast action stage as Sonic.
Super Sonic: Finish the game as Sonic, Tails, Knuckles, Amy, Big and E-102 Gamma. Super Sonic will be avaliable only in Adventure Mode.

Unlockables
Super Sonic: Complete the game with every character

Unlock characters
Metal Sonic: Collect all 130 emblems and visit Trial mode and pick Action Stage. He will then be a playable character (Note: Requires Sonic Adventure DX Addon)

Upgrades
Ancient Light (Sonic): On top of a rock on Angel Island
Booster (E-102): In the Weapon's Armory on the Egg Carrier
Crystal Ring (Sonic): Walk up the steps in the hotel, press the two buttons and use light speed dash
Fighting Gloves (Knuckles): On a ledge over Big's house
Jet Anklet (Tails): In a hole in the ceiling in the sewer behind Twinkle Park
Laser Blaster (E-102): In the Restricted room on the Egg Carrier after it crashes
Life Belt (Big): In the entrance to Icecap
Light Speed Shoes (Sonic): Find it in the sewer (not the onebehind Twinkle Park)
Long Hammer (Amy): Beat your own high score on Hedgehog Hammer after the Egg Carrier crashes
Long Rod (Big): Under the bed at Big's hut
Lure Upgrade 1 (Big): In the sewer behind Twinkle Park
Lure Upgrade 2 (Big): Under an sheet of ice in IceCap
Lure Upgrade 3 (Big): In a cave in the forest
Lure Upgrade 4 (Big): In one of the jail cells on the Egg Carrier
Rhythm Badge (Tails): In the flashback with Tikal
Shovel Claw (Knuckles): In the tunnel inside the tunnel where you find the cart to the Mystic Ruins Chao Garden
Warrior Feather (Amy): Beat Eggman's high-score on Hedgehog Hammer