Enter the following codes under Bonus items menu for the effects in Free Race mode: 

MUSCLE.....Extra power
MOON.....Low gravity 
BACKWARDS.....Mirrored tracks
STRETCH.....Double height suspension 
ONE2MANY.....Blurry motion
SHINY.....Chrome cars
NITRO.....Nitrous oxide booster
DRIVER.....70s-style springy suspension
KABOOM.....Exploding kerbs
BRICK.....Invincible car
DOTTY.....Tough bonus cars
Weflipped.....Mirrored Tracks
Twinpeaks.....2x Height
Ethanol......Speed boost
Glycerine.....Nitro boost
T2.....Chrome cars
Krazykerbs.....Exploding kerbs
ETHANOL.....Motion Blur
VANISHING......70's Springs
KERBKRAWL.....Exploding Kerbs
MADCAB.....Invincible Car
RAILROAD.....Cars drive like slot racing no steering needed.
EUROPA  .....Low Gravity
IAMTHEGAME.....All Tracks
fleeting.......All cars
GRUNTSOME .........Extra power

All Bonus Cars:
Type alldacarsalldatime in the password section in options to get all bonus cars.