These tips for getting more play time out of the Driver Demo comes from Ludwig:

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I'm sure everyone's already read about changing the demo mission to one of the others in Driver Demo:Scripts:Missions. The results of some tinkering with the mission text files themselves follow.

Add any of these lines to turn on the expected effect:

Night
Raining
Lightning
Foggy

Some of these might be exclusive of others (Foggy & Lightning, for
instance; haven't tried all combinations.

You can also drive any of the 18 cars from the full version by adding
(or changing) "PlayerCarType" followed by a number from 0-17. They're mostly missing their own engine sounds, though. Notables are:

1 - pickup truck
10 - LAPD cruiser (use the horn)
13 - NY taxi
16 - 174 MPH Jaguar

Have fun

You can also disable vehicle damage. Change EnablePlayerDamage to DisablePlayerDamage the dms file.
While your car will get banged up, and the damage meter shows damage, you cannot wreck your vehicle, unless you flip the car, which is an automatic end of game.