Successfully complete Tails' driving quest in the Hero side story to unlock the kart racing mini-game in the Hero side story. Successfully complete Rouge's chase mission in the Dark side story to unlock the kart racing mini-game in the Dark side story.
Successfully complete the Hero or Dark side story modes to unlock boss attack mode. You will be able to fight the bosses from the side of the story you completed.
Successfully complete the Hero and Dark side story modes. Then, enter story mode and a "Last Stage?" selection will appear on the menu between the Hero and Dark selections.
To get a 3D version of the Green Hills Zone stage, from Sonic The Hedgehog, you must get all the Emblems, including Kart, Timed Boss, and the emblems for all "A" rating. Once you have them all, which should total 180, the island next to the Green Forest stage will have the face of Sonic on it.
Successfully complete all of Sonic's stages and missions to unlock him in the PSO suit in Kart mode.
Successfully complete all of Shadow's stages and missions to unlock him in an alternate costume in kart mode.
Successfully complete all of Tails' stages and missions to unlock him in the Tornado 1 in kart mode.
Successfully complete all of Eggman's stages and missions to unlock him in his alternate Eggwalker in kart mode.
Successfully complete all of Knuckles' stages and missions to unlock him in his alternate form in kart mode.
Successfully complete all of Rouge's stages and missions to unlock her in an alternate costume in kart mode.
Successfully complete all of Sonic's stages and missions with an "A" ranking to unlock Amy Rose in kart mode.
Successfully complete all of Shadow's stages and missions with an "A" ranking to unlock Metal Sonic in kart mode.
Successfully complete all of Knuckles' stages and missions with an "A" ranking to unlock Tikal The Echinda in kart mode.
Collect all 180 Emblems to unlock the following hidden multi-player characters:
- Racing: Hero
- Space Sonic
- Racing: Dark
- Space Shadow
- Mecha Sonic
- Shooting: Hero
- Space Tails
- Shooting: Dark
- Space Eggman
- Hunting: Hero
- Space Knuckles
- Hunting: Dark
- Space Rouge
To make a Chao a Hero, simply use a Hero character and keep petting them. If done long enough, it will go white and be a Hero. To make it Dark, just use a Dark character and do the same thing.
Raise a Dark Chao with one of the three Dark characters. Continually feed and train the Dark Chao and it should evolve (creates a tear-shaped "cocoon"). Once it evolves, descending stairs should appear at the main Chao lobby leading to the Dark Garden.
Raise a Hero Chao with one of the three Hero characters. Continually feed and train the Hero Chao and it should evolve (creates a tear-shaped "cocoon"). Once it evolves, ascending stairs should appear at the main Chao lobby leading to the Hero Garden.
Get a good Chao and a bad Chao to unlock the good and evil Chao races.
Find one of the Skeleton Dogs; the easiest place to do so is the Pumpkin Hill stage with Knuckles, and combine it with your Chao. With either Knuckles or Rouge, start digging around in the Chao Garden. Eventually a pumpkin will appear. If you give it to the Chao mixed with the Skeleton Dog it will put it on its head. If you want to take it off, either throw or hit your Chao.
To wake a sleeping Chao, stand next to it and whistle. You do not have to anger them by picking them up during a nap.
Go to a Chao Garden and get a coconut. Then with a sleeping Chao, pick up the coconut and put it by the its head. The coconut will act as a pillow.
At the start of the City Escape stage, instead of landing on the ground with the board, scrape the roads on the board. When on foot run full speed at a wall and get knocked down. Pause the game if you fall after hitting the wall. Choose to restart the stage to run on foot and not on the board.
In the Security Hall stage you play as Rouge The Bat and are given 5 minutes to complete it. This can be quite difficult. If you have enough lives you can do this trick. If you cannot seem to find the Emeralds, intentionally die. You will keep the existing Emeralds that you have collected. You will begin at the start of the level and the Emeralds will move to different places. This is the fastest method for completing the stage if you are having trouble.
In Knuckles' Pumpkin Hill stage, go to the one of the train gates. You can see Big The Cat hiding behind the gate, waving to you. In the City Escape stage, Big The Cat appears after the last corner in the G.U.N. truck vs. Sonic scene. Look for him at the right of the screen, running downhill on the sidewalk away from the truck. In the Tails stage where you are running from the police and you first get your hover ability, Big The Cat can be seen again. There is a part in the level where you need to knock a column over in order to use it as a step to reach a higher level, from which you hover over to a platform with a police barricade and two robots. If you miss the platform and fall in the hole, you will see Big the Cat sitting on the girders underneath the platform. You can also see him in the battle against Sonic while playing as Shadow.
In Rouge's Inside the Pyramid stage, one of the robots you blow up is E-102.