A friend, who raised hogs at the time, once found a very thin strand of copper on his driver's seat. When he tracked it down, it grew insulation and went to the ignition coil.
When he found out which friend gave him that gift, he filled the friend's washer bottle with some boar urine that he had kept in a milk carton in the sun for a while.

I don't treat my friends this way, but:

Get some brass tire valve caps with rubber seals in them. Remove the valve core and quickly screw a brass cap on it. Loads of fun the next time he checks his tire pressure.

If he doesn't have remote entry, super glue does wonders for his door and trunk locks.

Butyric acid in the cowl vents. Smells like a yak vomited in the car. Only way to totally cure it is to replace the car.

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'79 300SD W116
'86 190E 2.3-16V W201
'92 300D 2.5 W124
'99 E320 4MATIC S210