I've narrowed it down to the sweet potato experiment.

For the sweet potato experiment, the independent variable would be the distance between to sweet potato plant and the other plants. the dependent variable would be how the growth of the plant at a certain distance. i would conduct this experiment by planting a sweet potato plant and next to it, in a line, planting the other plants and seeing how far away the closest growing one is to the sweet potato plant.

I looked into how i could find out how permanent, permanent markers are.
I think i could experiment this by putting the same type of permanent marker on the same sized pieces of cardboard and put them into different solvents like water and detergent.
i would keep the experiment controlled by having the same amount of each solvent, same sized cards, same marker and same length mark.
I would test how fast the reaction is for each test and the independent variable would be the solvent i use. the dependent variable would be the length of time it takes each test to react to the solvent.

Another idea i have had is to find how are plants affected by the distance between them.

I researched and found there is a concept of allelopathy. this is when plants take the growth of plants near them. They release chemicals called allelochemicals.
i found that sweet potatos are an example, so i thought i could find out how close other plants can grow to a sweet potato plant.

I am thinking about doing an experiment involving hairspray. I want to find which hairspray holds hair in position for the longest.
There might be alot of variabes , like how the different types of hair could effect the time.
I dont know where i could find the hair i need.
I would keep the experiment controled by using the same type of hair for all the hair sprays but i think it would be difficult to control how much of each spray is put into the hair. the controlled variable would then be hair with no hold in it.