Easy: CircuitPython running on an RP2040 or RP2350 microcontroller. Being able to write a simple bit of code and have it control something in the real world is sooooooo satisfying. OMG: You'll jump for joy the first time you get it controlling some LEDs, a display, a servo, a motor, etc.
Programming a microcontroller for the first time was definitely the most fun I ever had when programming. CircuitPython makes it so easy, too! Oh if only it had existed back when I started learning electronics.