How to lower the RPM of a motor without using PWM or gears?
The speed of DC motors depends on voltage, the current is proportional to the torque. If your mechanical load is very low, you will have low current, and the voltage drop in a resistor will be also low, so no change in speed. You can try with diodes, if the current is not very low, you will get moreless 0.5 volts with each one.
Other trial should be to use a resistor voltage divider.
Any how the speed control will be very poor.