Our team's final project is SmartKeys, a virtual keyboard over bluetooth. The keyboard is implemented as an android application which transmits keypresses to a remote Arduino Uno which then plays the appropriate note.
The decision to use Arduino over Mbed was mostly based on the presence of already developed libraries for the bluetooth module we obtained (adaFruit BLE UART Friend). The code for our code and the bluetooth library can be found from the main page (the other code comes with the Arduino IDE).
The code running on the android mainly consists of 4 files (the rest are just default files from initializing a new android project in Android Studio). These are the manifest, which specifies some app-wide attributes such as fixed orientation, the xml for the main app view which simply includes the Piano view, the Piano view which handles drawing and keypresses, and the code for the main app view (MainActivity) which initializes bluetooth. These main files can be viewed from the main page, and the entire project, which can be imported into Android Studio to be built, is available here.