If you want to get into electronics, one great thing to have is some modules/sensors lying around for fast prototyping. You can buy cheap 'kits' of sensors (Search '37 sensors arduino' on banggood or aliexpress) with tilt, touch, laser, joystick and so on. A good write-up of the common ones can be found here: https://tkkrlab.nl/wiki/Arduino_37_sensors

My loving brother bought me a kit - here's my first useful hack with it as an example of how easy this sort of thing is. I was reminded why so many people choose arduino for prototyping - this whole project took about half an hour. Sure, it's not energy efficient or as cheap as it would otherwise be, but I'll re-use all the components and I didn't even have to solder a single wire.

The goal was to make a box that would play a melody and blink some lights whenever someone recycled a plastic bottle. I thought of having the bottle break a beam, or activate a vibration sensor when it landed, but I decided to go with one of the little mercury tilt switches.

Without further ado, here's the code:

/*
 * Sketch modified by Jonathan Whitaker
 * Based on example code by by Tom Igoe http://www.arduino.cc/en/Tutorial/Tone
 * All code public domain.
 */
#include "pitches.h" // Copy from arduino tone example for this to work

// notes in the melody:
int melody[] = {
  NOTE_C4, NOTE_G3, NOTE_G3, NOTE_A3, NOTE_G3, 0, NOTE_B3, NOTE_C4
};

// note durations: 4 = quarter note, 8 = eighth note, etc.:
int noteDurations[] = {
  4, 8, 8, 4, 4, 4, 4, 4
};

int tiltPin = 2; // Signal pin of tinlt sensor
int led = 13;

void setup() {
  pinMode(tiltPin, INPUT);
  pinMode(led, OUTPUT);
  digitalWrite(led, LOW);
}

void loop() {
  if (digitalRead(tiltPin) == HIGH) { //read tiltPin     
    digitalWrite(led, HIGH); //turn LED on
    play(); //Play a tune
    digitalWrite(led, LOW);
  } 
}

void play(){
  // iterate over the notes of the melody:
  for (int thisNote = 0; thisNote < 8; thisNote++) {
    int noteDuration = 1000 / noteDurations[thisNote];
    tone(8, melody[thisNote], noteDuration);
    int pauseBetweenNotes = noteDuration * 1.30;
    delay(pauseBetweenNotes);
    noTone(8);
  }
}

Simple and easy, especially since I just modified the example tone sketch. SO one wire of the sensor goes to pin 8, the others to power and ground. I also have the option of RGB LED blink patterns, but I'll leave that out of the code for simplicity. Here's a photo of the box (from inside):

As a bottle is pushed through the hole it bumps the sensor (which has since been hot-glued in place) and the barbers knock song plays while lights blink. Simple, easy and fun - the perfect beginner project.