Five upgrades with deliberately silly names, a click counter, and nothing else. Frobnicator, Alpha Spanner, Painted Turbine, Geared Lutefisk, Tiled Barnacle: buy them in order and the counter goes up faster.
It was written as a worked example for a blog post about building an incremental with RxJS, and it plays like one. Ten minutes exhausts it. Take it as a demonstration of the loop at its smallest, where every part is visible at once.