I was thinking about car diagnostics.
Connect the RasPi to my car's 12v charger plug, connect wirelessly (Bluetooth) to both an OBD-II (onboard diagnostics) and GPS device. Constantly log data (lat/long, car speed, gps measured speed, rpm, mpg, battery voltage) to the SD card.
When I come home, I hook the RasPi up to my TV using HDMI and use a WiiMote to control it. There I can see a map of my journey, I can zoom in and "replay" my trip whilst watching the logging data at that exact moment. So I can see myself on the map driving, with a speed readout, tachometer, which gear I was in, MPG etc.
Might by pointless to many, but I would find it marvelous.
I already have experience with NMEA (GPS protocol) and OBD-II (diagnostics protocol), Wiimote is also easy to me. The only thing I have no experience with is building a GUI in linux, that will be the hardest part. I might take a shortcut by doing a web-interface and a fullscreen browser.
Plus, the parts are cheap. €20 for both the GPS and OBD-II unit combined, another €10 for a Chinese Wiimote, another €3 for a USB bluetooth dongle.