The first step in deciding which is your best idea, you MUST understand that your science fair project has to be an experiment. What that means is that it cannot be a demonstration of a process in science like volcanoes or tornadoes, it must be about doing an experiment. So... what makes an experiment?
1. What is your independent variable?
Now, you must have three variations of this variable. For example, look at the image to the left, you will see three different variations of each independent variable. Amount of salt or the height of the ramp are the two independent variables. So...
2. What are your three independent variables?
Now, think about your independent variable you described in questions 1 and 2. How are you going to measure how these variables will be affected in your experiment?
3. What is your dependent variable?
4. How will you measure this variable?