Cut your scrap lumber into two pieces -- one piece that is 1 foot long and another that is 6 inches long. Use the jigsaw to cut a circle or square large enough in the shorter piece to accommodate the small electric motor.
Use a hammer and nails to connect the longer piece of wood to the shorter one at a right angle. Insert the motor into the hole you made and attach the propeller to the bottom.
Connect the leads from your multimeter to the positive and negative terminals of the motor. Test your connections by turning the propeller with your hand. If you spin it quickly, you should see as much as 0.6 volts on the multimeter.
Take the apparatus to a stream and set the propeller down into it. Watch the multimeter's readout to see the voltage that the water is generating.