zip ties, pipe, caps and a ball bearing. Place it on the frame of their car, either inline or perpendicular. when they accelerate/brake or turn respectively they hear "clunk"
Zip tie on the drive line. THUMP THUMP THUMP as they accelerate.
I did one at work where we had a STACK of wireless door bells, i set them to the same channel and hid them all over the office, kept the clicker in my pocket.
Sticky mouse traps, just put them on the floor when you know they'll walk by, it's hell getting them off.