The problem with the "retro" telephone ring sound that so many modern phones have built in is that it just doesn't resonate and echo and slowly decay over time like a real phone ring. It would be cool to code up a physical ring simulator that reproduces that sound properly, and responds to the accelerometer, especially when you drop it or hang up on somebody by slamming it down.
https://www.youtube.com/watch?v=AxXsIQDafog