Air fresheners such as those little trees are wonderful if you slowly peel them open, but they are horrible if you peel them all the way open at once!
So that is exactly what you will do to your victim. Unpeel a tree or whatever you choose and hide it somewhere in their car. Wedge it under a seat, hide it in the dash, someplace where it is going to be hard to find.
Pine scented is particularly aromatic.
If you cannot unlock the car, slip it into a window or wedge it under the car. Even better, put the air freshener in the space where the front window meets the hood of the car. This is the air intake for the air conditioner. The smell from the air freshener will permeate the car through the air vents.