Don't key cause they already did that, now you have to go above and beyond.

I'd do 3 out of 4 tires. If you do all 4 the insurance could cover them under someone screwing with their car. Do like 2 or 3 and you may be good. You need them to come out of their pockets. And if you do the tire do it right, cut the stem off. That way no patching or plugging will help they'll have to purchase new tires.

Sugar in the gas tank.

Holes in the muffler.

Destroy electrical equipment will work as well, just need to find a way to get under the hood.

If you gonna do it, do it right.