To enhance and keep up with the other "freemium" tracking apps out there, I think it would be worth while adding auto trip start functionality as a premium feature. Once the device connects to a specific Bluetooth device, it adds a new trip record.
I know some go into more detail by tracking distances, but I am not sure this is needed. If the trip exists, you can go back and add/adjust mileage. This could also be coupled with my previous suggestion of OBD II support where it will automatically record the start and stop odometer and the beginning and ending locations.