I think I must be a little boring, but, I would scrub a nail/screw head or 2, quite suitably with a sander, before hammering it into a tyre or 2...
I would also get a suitable length of wire and a bulb connected to the altenator and suitable spot on the body, to drain and keep draining the battery (Imagine their shock when they take it to the garage and discover the problem at hand... Better still if they have replaced a battery)... Done nicely, could keep a few mechanics guessing (Especially the Stealers)...
I would run some expanding foam up the exhaust, then drill a hole through it, so the engine starts and runs a low revs (would work very nicely with some black spray over the tail of the foam, to conceal it, same issue off to the mechanics, then they have to prove that you did it)...
Gaining access under the bonnet would open the door to all kinds of fun, such as polyfilla in the water and sand in the oil...
If you don't want to touch the car, a couple of large concrete bollards stuck behind it would give some you tube fun... especially if you set them 1 inch smaller than the width of the car apart...
Humiliate the b@st@rds as much as possible... Thats my viewpoint...