I just got a Pi. My version is the Pi 2B. I have only played around with it once, getting basic C programs to compile and run. I have an old beater car that I am think of turning into a Pi-dev car. I have all kinds of ideas. I'm thinking of doing a laser range-finder to track following distance, maybe a license plate scanner. Building a GPS/INS would be interesting as well. Or maybe a dash-cam that links video to a remote server (over WiFi->G4). I don't drive in really corrupt places but say you were driving in Russia or Mexico and the cops pulled you over and hassled you. When they find you have a dash cam, what happens? Crunch. But you got the video back home...
For starters I'm gonna play around with doing basic multi-core programming at the C or even assembly level. I've never played with ARM before, it should be fun.