Pinewood Derby Car
A model car full of blinkenlights that I programmed when I was a smol child.
A quick video of the car in action.
When I was a smol child in cub scouts, we had a competition to see who could build the fastest car. I didn't really care about that, so I decided to try and make something cool instead using an Arduino I had recently gotten.
Almost everything is wired directly up to the Arduino, except the LED matrix, which runs off of the I2C bus. I modified some example code to display an animation of flames scrolling across (that I had drawn up on some graph paper).
I think I won an award for it? (To be honest it's been so long that I can't remember well). But I do remember the battery dying - I had used a small garage-door-opener battery in order to fit within the weight limit, and that ended up dying pretty quickly.