How about covering a car with Post-it notes?:
You also have the driverless Minivan prank:
There is the traffic snarling invisible rope trick:
Stuffing their car with golf balls is always funny:
How about the time-honored faked accident?:
You can just hide the car altogether:
There is the zip tie car prank - with complete instructions:
There is also the fake blown tire prank:
[Source: via MotorAuthority]