Here I just made a simple little piano. It uses four push buttons connected to digital pins 1-4 and a piezo speaker connected to pin 8. I used notes that are easily distinguishable when played.

 

the code:

 

#include "pitches.h"        // include this library

int switchPin1 = 1;          // set up variables
int switchPin2 = 2;
int switchPin3 = 3;
int switchPin4 = 4;

void setup() {
  pinMode(switchPin1, INPUT);        // set pinModes to input
  pinMode(switchPin2, INPUT);
  pinMode(switchPin3, INPUT);
  pinMode(switchPin4, INPUT);
}

void loop() {
  if (digitalRead(switchPin1) == HIGH) {        // if button is pressed,
    tone (8, NOTE_D3, 50);                          // play this tone
  }
  if (digitalRead(switchPin2) == HIGH) {
    tone (8, NOTE_G3, 50);
  }
  if (digitalRead(switchPin3) == HIGH) {
    tone (8, NOTE_C4, 50);
  }
  if (digitalRead(switchPin4) == HIGH) {
    tone (8, NOTE_F4, 50);
  }
}

 

A Fritzing picture of my circuit is attached.