Would it be possible for you to build a Heads Up Display that shows either:
1. Directions
2. Maps
3. Twitter feeds
4. Facebook Updates
5. Traffic information
or whatever using shop built machinery. The idea is that it is displayed infront of you on the windscreen and I guess you'll need the device to have a built in GPS to identify your location.
I guess you can include your phone in the build so that instead of purchasing a whole new item to have gps and internet access (for the twitter / facebook feeds) so in effect your challenge is to integrate the phone with build.
Extra points if you can make your HUD touch sensative (so that touching your windscreen causes say a switch between apps.