I need any science experiment with an Independent Variable and a Dependent Variable, and of course an experiment that can be done by a Highschooler...

Uh..it’s not that interesting, but a simple one is to measure the growth of two plants. Take two seeds of the same plant and give them the same amounts of water and all that every day, but keep one near a window and one in an area with little-to-no light.

Since you can measure conductivity, how about starting with a liter of water and a bag of salt.
Keep the probes a fixed distance apart and keep adding a small amount of salt (the independent variable.) and measure the resistance (the dependent variable.)

Answer the question why is salt water more corrosive?

I did one about which nail polish brand/line was the best, although I can’t remember what the independent/dependent variables were at the moment.

Your example involves physics. Another physical science experiment is to test the distance a spring stretches verses the weight of an object hanging from it.

Hmm, there are many that I could come up with, but if you look into a specific field of science, you’ll come up with a few great ideas.

Here’s a favourite from Calvin and Hobbes about how they test the weight limit of bridges…
http://www.s-anand.net/blog/calvin-and-hobbes-dad-explains-science/

“They drive bigger and bigger trucks over the bridge until it breaks. Then they weigh the last truck and rebuild the bridge.” The independent variable would be the weight of the trucks, while the dependent variable would be the bridge breaking (or not). The controlled variable would be the bridge.

Your independent variable is different materials. Your dependent variable is conductivity.

Or your dependent variable is opacity or something.

You just need a theory, like heavier materials block off more light, or heavier materials are less conductive—really, any theory at all. It can be wacko. You just have to have the means to test it.