This is a project where testing and data gathering is necessary. All experiments need several different trials with a control and a variable. As you are conducting your experiment, make a real-world connection. Why is the experiment you are conducting important to the world we live in?
What is the effect of detergent on the growth of plants? Which paper towel is more absorbent?
What structure can withstand the most amount of weight?
Here are some places where you can look for experiment ideas:
Some of these ideas are demonstrations meaning, it shows you something that happens. Make it into an experiment by changing the amount of certain materials used or modifying one aspect of the demonstration to see if the results are also different.