Sonic Adventure cheats and cheat codes for Dreamcast

Mega Characters:
Win the game with every character, including Super Sonic. Return to
the character selection screen. Now whatever character you select
will be "Mega" - Mega Sonic, Mega Tails, etc. They'll now be much
faster and invincible. 

Unlock Super Sonic
Win the game with all 6 starting characters

Play as Tails
Use controller 2 to control Tails

Full pause screen                                                 
Press X + Y, while paused the gameplay.

Secret images:
If you insert your game disc into a PC CD-ROM drive, you will find 
lots of images from the game.

Change Twinkle Park Race views
Press Up during the Twinkle Park Race stage