I'm pretty sure this has been done in the past, but a friend of mine's family pulled a fast one on their 80-some-year-old grandpa.
He drove to a store or restaurant, and while he was inside, they went to his house, got his spare keys and "stole" the car. The best part is that they set up everything with the cops beforehand, so when their grandfather called, they said they'd look for the car and sent him on his way.
Pretty ingenius.