If you use Arduino to create a MIDI controller, there's no limit into what the controller's interface should be. You could use standard buttons and sliders, or distance and motion sensors. Anything that can be used to create a signal could possibly be part of the controller. Here we have such a project... Lucas Zanella has created a piano using nothing but coke cans.
Each can is connected via a wire to a BC548B transistor, which basically amplifies the signal from the body when the can is touched. The Arduino reads the signal from the transistor, and converts this to a note, which is sent out over a serial connection to his MacBook.
A Python script on the computer reads the serial data from the Arduino, and reacts to play the various audio loops. This could easily be adapted to support a standard MIDI interface, so you can control any synth with the coke cans. This is quite a clever idea, and you could use any collection of metal objects in place of the Coke cans. You could, for example, use a glockenspiel as a MIDI controller or a set of spoons.
Lucas has provided the Python code for the build so you can see how the audio is triggered, however he has not uploaded any tutorial or the Arduino code. The Arduino code should be relatively easy to work out if you already know how to read and write to the pins and serial connection.
Check out the video below.