The final project is where you can be creative with the Arduino. Try new sensors or output devices or create interesting software. I base my grade both on creativity and technical difficulty. For example, using a range finder and printing distance to the Arduino window on your laptop is probably worth a grade of 80. Using a range finder and displaying distance on an LCD is probably a 100. A mostly working mobile vehicle is at least a 90 — probably closer to a 100. a car with two photosensors following a light source sounds like a 100. A tilt sensor controller several LEDs is 80. I hope these examples give you some idea of the level of effort I expect.