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

howhardcanitbe0 : Difficulty adjust (0 = easiest)
howhardcanitbe1
howhardcanitbe2
howhardcanitbe3
howhardcanitbe4
howhardcanitbe5
howhardcanitbe6
howhardcanitbe7
howhardcanitbe8
howhardcanitbe9 (9 = most difficult)

starpower : Gun (key F by default) and no damage
leatherandlace : Gun, rocket launcher (keys F and H) and no damage
upupdowndown : Unlock all cities, vehicles and modes (arcade only)
theparrot : Unlock all modes
griswold : Unlock all cities
octane : Nitrous
yerbamate : Unlimited Nitrous
rimbuk : Unlock all cities, vehicles and modes (arcade only)
starlite : "hallucinating view"
tumbler : ?
aquaspark : ?
happyhour : ?
operationdoom : ?
quickFix : ?

howfastcanitbe0 : Speed adjust ?
howfastcanitbe1
howfastcanitbe2
howfastcanitbe3
howfastcanitbe4
howfastcanitbe5
howfastcanitbe6
howfastcanitbe7
howfastcanitbe8
howfastcanitbe9