Cheat mode
Pause the game, hold shift and type:

Code - Result:

S66S - Fat Tires
4awd - Perfect Balance
4w62 - Add 8 Minutes to Run
648w - Unlock Burnside
648a - Unlock School
648d - Unlock Warehouse
648s - Unlock Granny
as86wa84 - Always Full Special
466wss - Score X 10
ssw664 - Score / 10

Best bike
Win gold medals in both competition levels to unlock your rider's best bike.

Sprocket Stall:
There is only one place where this can be done, and it is the only rail in the level. Go there and grind on it where your bike is completely crooked by the wheels are the opposite way of the rail. You should stall if done correctly.

Tony Hawk (alternate):
Get all thirty covers and two gold medals in career mode with any character to unlock Tony Hawk on an old girls bike.

Dirt track:
Go to the half pipe where it goes to the small place right next to it. Go over that long pipe (over the whole track) and grind it. The garage door will open, revealing a dirt track on the other side.