Courses:
Hold X + Y at the options screen. Select the "Password" option that will appear. Enter one of the case-sensitive passwords to unlock an additional course:

Cheat             Course
CinqueValvole     Fiorano course.
LiebeFrauMilch    Nurburgring course. 
Stars&Stripes     Laguna-Seca course. 
KualaLumpur       Sepang course.
DaysofThunder     Atlanta course. 

Full Pause Screen:
Press Start to pause the game and hold X + Y.

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Hints

Link Cable?:
Press X+Y on the main menu screen to access a link cable versus race option. Note: The link perripheral for the Dreamcast was only released in Japan. So finding one will take a little searching.

Useless But Funny:
Go into one player mode and pull into the pits. Wait until all four wheels are off, pause and quit. If the results screen shows a stationary car, it won't have any wheels on! Useless but funny!

 

Thanks to Revolution readers Mario, Berger2K1 and Joshua!