A GPS tracker for your car, motorcycle, bike and flying saucer, with data storage and map visualization. Not too hard, but no easy either...
Automated parking system using RFID and Ultrasonic sensors.
Internet controlled Hovercraft
A DeLorean security system to prevent uninvited drivers.
A mesh of open devices monitoring railway crossings "open-closed" status.
Let your engine breathe a little easier.
An f1 inspired smart room controller that involves a race between pixels or freinds.