One of my favorite weekend projects was an autoclicker. I had (have?) an intellectual infatuation with incremental games, and just found them to be incredibly alluring. I already knew how to write an auto clicker in software, using either existing software, or hooking into processes with my own functions. But, I'd never done any hardware modification for something like this. I had an old mouse, and some hardware kits for arduino/raspberry pi, so I had a 555 timer, and a switch, and a soldering iron, and a drill. And that's all I really needed. I took apart the mouse. I did some calculations, and designed my timer circuit. I soldered the button onto the circuit to click the mouse. Then I tuned the resistors to get enough clicks per second that the particular game I was playing would register them all. Then, I drilled a hole into the mouse case to give me easy thumb access to the button.
It was glorious. I could just hold down the button, and watch numbers go brrr. Sure, I could have bought any number of mice to do it for me. But I wanted to do it, and it was fun, and I have something to show for it. Someday, I may even take it apart and add a rheostat so I can adjust the click frequency. It was a simple hardware project that helped jump start my hardware/firmware career.