How to lower the RPMs in small a DC motor
Theoretically, it's possible, but in reality, you won't be able to without your own control circuit, or doing something that might damage your motor.

The reason for this is because standard DC motors will require a minimum amount of power to operate.

Here's a section of a datasheet for a 3V-4.5V DC motor:



At the graph in the bottom, you see that the motor speed (in RPM, symbol N), there's pretty much no way you're getting it down to 5-10RPM, unless you can change physics.

A couple of things you can do:


Use gears to change ratio of speed, which is what you're going to do. This is probably the most simple, cheapest, and safest option, in my opinion
Use a stepper motor, which are commonly used for high-torque, low RPM applications
Find some sort of PWM control circuit to slow it down, although you probably won't be able to get it down to 5-10RPM


Things you shouldn't do (you probably already know this, just leaving this here for others):


Use some sort of physical restriction to slow it down (i.e. weights). This is bad. It could burn out your motor, and put a lot of stress on your power supply (if it's not mains), and even if it does work, you're going to be running very close to stall current for a long duration of time, which is wasting A LOT of power.