I have an implant in my foot and I use it to log into my computer at work… and perform cheap magic tricks, exactly as you describe 
As for the car, I plan on having it start up when I depress the clutch (NFC reader under the pedal). Since the implant is in my right foot, it can’t be operated by mistake, as it takes some effort to work the clutch with the wrong foot, obviously.
Trouble is, the damn car won’t start without the 433 MHz transponder in the original key, and I don’t have time to bypass it at the moment. So for now it’s just a plan.