Taking an unstable DC voltage, and stabilizing it
The absolute simplest possible solution is a capacitor (high value electrolytic, greater than 5V - probably at least 10V) across the output terminals.

However, there are two problems with that: it will attempt to drive the generator backwards somewhat, and it merely smooths the voltage rather than regulating it to a particular value.

To solve the first problem, add a series diode (possibly Schottky) from the generator to the capacitor.

To solve the second problem, I would add a DC-DC buck/boost converter module to the output. You may be able to get this counted as a single "part". In order to prevent the capacitor voltage increasing without limit if there is no load, for small values you can use a Zener across the capacitor (some value higher than 5V but lower than the capacitor's voltage limit). For large generators you will need to build a dump load, which is a resistor that gets switched in when the generator is producing more power than required. Or feather the propellor.