Day 12 of Learn Coding with Arduino — hook up an SG90 servo motor, sweep it from 0° through 180°, then steer its position live with a potentiometer knob.
Day 15 of Learn Coding with Arduino — hook up a 16x2 LCD over I2C and display your first line of text. Your Arduino finally gets a screen of its very own.
Day 19 of Learn Coding with Arduino — hook up an IR receiver and drive LEDs with any TV or DVD remote in the house. Your first taste of wireless input.