Go to Options/Cheat Codes and enter one of the following codes : 


tumbler ?
aquaspark ?
starlite "hallucinating view"
howhardcanitbe0 Difficulty adjust Easiest
howhardcanitbe1 Difficulty adjust
howhardcanitbe2 Difficulty adjust
howhardcanitbe3 Difficulty adjust
howhardcanitbe4 Difficulty adjust
howhardcanitbe5 Difficulty adjust
howhardcanitbe6 Difficulty adjust
howhardcanitbe7 Difficulty adjust
howhardcanitbe8 Difficulty adjust
howhardcanitbe9 Difficulty adjust Hardest
starpower Gun (key F by default) and no damage
happyhour ?
leatherandlace Gun, rocket launcher (keys F and H) and no damage 
operationdoom ?
upupdowndown Unlock all cities, vehicles and modes (arcade only)
theparrot Unlock all modes 
griswold Unlock all cities
quickFix ? 
octane Nitrous
yerbamate Unlimited Nitrous
rimbuk Unlock all cities, vehicles and modes (arcade only)
howfastcanitbe0 Speed adjust Easiest
howfastcanitbe1 Speed adjust
howfastcanitbe2 Speed adjust
howfastcanitbe3 Speed adjust
howfastcanitbe4 Speed adjust
howfastcanitbe5 Speed adjust
howfastcanitbe6 Speed adjust
howfastcanitbe7 Speed adjust
howfastcanitbe8 Speed adjust
howfastcanitbe9 Speed adjust Hardest