Hey Mario, I think it's great that your wanting to do this for your science project.
I'll tell you what I did for my 8th grade science project. It's not really electronics related. I made a wind tunnel (I was into aerodynamics and airfoil shapes and how they work). I basically made a box, with a transparent top. One end of the box had a fan, the other end was open to allow the air to flow through. I made several airfoil shapes (airplane wings) out of balsa wood, and tested them in the wind tunnel. I basically observed which airfoil shapes worked the best under different wind speeds. Which ones vibrated the most, which ones provided the most lift, which ones didn't work etc. The variables were the wind speed, and the different airfoil shapes. Just an idea, if you're in to aerodynamics
Because you seem to be interested in magnetism...You could build a generator to light a small light bulb, or a row of light bulbs. My sister did this for her project. I'll post a link or two on this, once found. The variables in this case are the RPM's of the shaft, and the output voltage and amperage.
OR you could build a wind generator. Basically the same as above, but the generator is turned with a fan blade from wind coming from a nearby fan.
An expansion on this project, you can use several different sizes of DC motors as generators. When the motor is not powered, and turned, the motor then becomes a generator. Thus, the motor will generate amperage and voltage, based upon the size of the motor and the RPM's that the shaft is being turned. You could show how different sized motors (Variable 1) and different RPM's (Variable 2) provide more or less voltage and current.
You could make a very simple audio amplifier, and show how different values of components will produce more gain (Loudness in the sound). This is a little more complex, but still able to be done.
You could build a solar power station (Although, this may become a little expensive) and use solar panels to light a small light bulb or LED. Variables here would be the incidence vector of the light rays (The direction that the light is hitting the solar panels) and How much light is needed to light a bulb or LED.
You can make a 555 timer circuit to make an LED flash. The variables here would be the oscillation (How fast the circuit will flash the LED) and the ON time (How long the LED stays lit according to the components used in the circuit. See link
**broken link removed**