I am replacing a small DC motor that powers a Santa mobile using single AA battery - the only thing I could find was on Amazon, by "ajax" - listed as 1.5-3.0 V. The motor spins the propeller a little too fast, and it seems like reducing the voltage to 1.0 V or so would give the correct RPM. If so, what is the best (easy) way to reduce it?
I'd start by putting a diode in series with the battery and see what you get. A ordinary silicon rectifier diode, like any of the 1N400x series will be fine. These will drop 600-700 mV, which might be enough to slow the motor down to the speed you want. If it's still too fast, try two diodes. It is unlikely that will be too fast.
The advantage of a diode versus a resistor is that it will drop the voltage a more predictable amount, regardless of the current being drawn by the motor. A resistor in series will work, in theory, but that requires knowing the current and voltage the motor draws at the desired operating point. The resistor will also get more in the way when the motor is trying to start up. It takes more current to start than to continue running once up to speed. However, a resistor will drop more voltage during startup, right when the motor needs it more.
Use a diode or diodes, 1N4004 for 0.7V drop, 1N5819 for about half that.
Using a resistor will not lower the no-load speed, it will lower the available torque (so the speed might drop when the motor is loaded, or it might stall).
A voltage divider will lower the speed but it will also unnecessarily lower the available torque, and it wastes a great deal of battery power.
A diode (or diodes) will almost maintain maximum torque at the lower speed without getting into the complexity of a feedback or IR compensated controller.