The canonical first thing to do with an Arduino is make LEDs blink. Let’s do it. I started with the Arduino Cookbook, which is excellent but not very deep, I gobbled it in a few days.
I created first a basic LED sequence, which was easy and I was delighted by.
The programming was so simple I looked for more. I adapted some code from the Cookbook and got a more interesting pattern and an RGB LED going.
This was a very satisfying set of hacks. The Arduino Wiring language is nearly C++, and really only C if you do simple stuff. Still, I live a lot higher in the stack (e.g., R, python, bash), so getting into bits requires me to think hard. That’s the point, of course, so it’s all good.