DIY generator. Simple and small
How to convert a motor into a generator?

Easy motors

If it's a permanent magnet motor, it's easy. Spin the motor, use the power, profit.

Brushed permanent magnet DC motors. Often found in toys, and old-style cordless drills and screwdrivers. These will produce DC. The gearing is often reasonable to simply spin an electric screwdriver via the chuck with your turbine.

If you buy a BLDC (brushless DC motor) these produce AC, which you can just 3-phase rectify from the output. These produce a (to me) stupendous amount of power per volume, as they use very strong magnets. This is what I'd use for a DIY windmill or turbine.

Mains synchronous motors like microwave turntable produce single phase AC, but at flea-power level, maybe worth a play.

Hard motors

If there are no magnets in it, it's hard. Spin the motor, provide excitation voltage, or specific starting and running conditions, waste of time if you're not already a motor/electronic engineer.

Induction motors - fans, washing machines
Wound motors - mains electric drills, food mixers, car starter motor