As a wind turbine, the voltage generated by a small DC motor has a certain proportional relationship with the speed. If the fan blade speed is slow, a set of gears must be installed to convert the low fan blade speed to the high generator speed.
Find an old electric toy, which usually has a gear reducer inside, and remove the motor and reducer together (do not separate the motor and gearbox). Install the fan blades on the impeller side. The motor shaft should spin very quickly and should be able to power a small 3V light bulb. I've played with it before.