Unlock Extra Music Track:
Go to the themes menu and turn the analog stick clockwise quickly until you hear a sound.

Big the Cat Appearances:
There are several places in SA2 that you can see Big the Cat from the original Sonic Adventure Dreamcast game. Here are a few of them:
1. City Escape: About 3/4 of the way down when you're running away from the truck look on the right sidewalk to see him running away.

2. Radical Highway: After going in one of the rockets look at the blimp to the left and he will be sitting on it.

3. Pumpkin Hill: Go to the area where the train is stuck (it's going back and forth) and he is behind one of the gates waving to you.

4. Cannon's Core: With Rouge, go to the top of the highest pillar in the main room and look up. He's jumping up and down.

5. When you're battling against Shadow with Sonic in the Green Forest area there is a grate. Look down in the grate and you can see Big fishing.

Boss Attack Mode:
To unlock the Boss Attack mode, complete either Hero or Dark side story modes. You'll be able to fight each boss from either Hero or Dark side.

Unlock Dark Garden:
To unlock the Chao Dark Garden, you must raise a Dark Chao with one of the three Dark characters. Continually feed and train your 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.

Unlock Green Hill Stage:
To unlock a 3D version of the classic Green Hill stage (from Sonic the Hedgehog on Genesis), earn all 180 emblems. The new stage will appear on the world map.

Unlock Hero Garden:
To unlock the Chao Hero Garden, you must raise a Hero Chao with one of the three Hero characters. Continually feed and train your 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.

Unlock Kart Racing game:
In order to unlock the Kart game, in the Hero Side story, complete the Tails driving quest and in Dark Side story, successfully finish the Rouge chase mission.

Unlock Last Stage:
In order to play the actual last stage of this game, finish both Hero and Dark side story games. Once done, go into story mode and a "Last Stage?" menu selection should appear between Hero and Dark selections.

Unlock Super Sonic and Hyper Shadow:
Everybody wants to play as Super Sonic, and now you can. All you need to do is complete "Last Stage?" in story mode, and you'll unlock both Super Sonic and Hyper Shadow.

Hint: Escape Time Stop:
In any 2P Battle, when someone uses Time Stop on you, push every button except START and wiggle the control stick as much as you can to make the time go faster.

Hint: Planting Seeds In Chao Garden:
The game doesn't really help with what you do with Chao. If you wondered how to plant a seed, read this. Beat all three levels of every chao race (or at least the Crab and Stump Valley races). Then go buy a seed and give it to the winning chao and it will plant the seed.