Disarm the car's alarm, and open the driver side door.
Turn the radio and windshield wipers on, and set the climate controls so the fan will come on when the car is started. This extra measure won't be noticed until the end, but it is a nice punchline to the prank.
Open the first bag of brightly colored packing peanuts and distribute its contents evenly over the floor boards in the front and back of the car.
Spread the contents of the second bag onto the seats and armrests before rolling down the driver side window. Close the car door and begin emptying the third and fourth bags as you reach inside to spread them with your arm.
Roll the driver side window up until there is only a 3-inch opening. Open the fifth bag of packing peanuts and push them in through the small window opening.
Raise this final bag of packing peanuts over your head and shake it as you hold the open end in place. If the packing peanuts are spilling out, seal the window with a piece of plastic wrap or clear tape.
Reset the alarm on the car and return the keys to the vehicle owner. Play it cool.