Answer is going to be dependent on how much access you have to the car, where it is located, whether or not you are concerned with being seen, and how visible it is to things like neighbors, security cameras, etc.
Things like sugar in the gas tank and bologna on the paint are urban myths. Google it if you want to verify on your own.
If you want a quick and easy way to cause serious damage to someone's car without attracting a shit ton of unwanted attention, invest in some airport paint remover. You can get it in aerosol form, but you want to avoid the noise of a spray can, get the raw product and a heavy duty paint brush or roller. The raw product will typically produce better results anyway. Just casually walk by the car while letting the brush or roller make contact with the paint.
Check
https://www.youtube.com/watch?v=xezcymF47aQ if you want to get an idea of the results, or just search YouTube for airplane paint remover.