Last stage
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.

Bonus stage
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.

Play as Sonic in alternate costume in kart mode
Successfully complete all of Sonic's stages and missions to unlock him in the PSO suit in Kart mode.

Play as Shadow in alternate costume in kart mode
Successfully complete all of Shadow's stages and missions to unlock him in an alternate costume in kart mode.

Play as Tails in the Tornado 1 in kart mode
Successfully complete all of Tails' stages and missions to unlock him in the Tornado 1 in kart mode.

Play as Eggman in alternate Eggwalker in kart mode
Successfully complete all of Eggman's stages and missions to unlock him in his alternate Eggwalker in kart mode.

Play as Knuckles in alternate form in kart mode
Successfully complete all of Knuckles' stages and missions to unlock him in his alternate form in kart mode.

Play as Rouge in alternate costume in kart mode
Successfully complete all of Rouge's stages and missions to unlock her in an alternate costume in kart mode.

Play as Amy Rose in kart mode
Successfully complete all of Sonic's stages and missions with an "A" ranking to unlock Amy Rose in kart mode.

Play as Metal Sonic in kart mode
Successfully complete all of Shadow's stages and missions with an "A" ranking to unlock Metal Sonic in kart mode.

Play as Tikal The Echinda in kart mode
Successfully complete all of Knuckles' stages and missions with an "A" ranking to unlock Tikal The Echinda in kart mode.

Multi-player characters
Collect all 180 Emblems to unlock the following hidden multi-player characters:

Racing: Hero
Sonic
Space Sonic
Amy

Racing: Dark
Shadow
Space Shadow
Mecha Sonic

Shooting: Hero
Tails
Space Tails
Choa

Shooting: Dark
Eggman
Space Eggman
Big

Hunting: Hero
Knuckles
Space Knuckles
Tikal

Hunting: Dark
Rouge
Space Rouge
Chaos

Hero or Dark Chao
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.

Dark Garden for Chao
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.

Hero Garden for Chao
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.

Good and evil Chao races
Get a good Chao and a bad Chao to unlock the good and evil Chao races.

Pumpkin head for Chao
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.

Wake a sleeping 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.

Chao pillow
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.

Run on foot in City Escape stage
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.

Quickly complete Security Hall stage
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.

Big The Cat appearances
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.

E102 appearance
In Rouge's Inside the Pyramid stage, one of the robots you blow up is E-102.