You could jailbreak and install the tweak AirPlayServer to stream music to it. Just plug it into a set of speakers or a TV.
Pick up a bluetooth OBD2 scanner for your car and use it to display things like gas mileage, engine RPMs, cam timings, or values from any of the other myriad of sensors your car has.
Looks like you can still get around $60 bucks for it on glyde if its in really good condition.