Here's a collection of SRP ideas that I found from past BoS posts and other websites you may consider
How will the size of the paper affect the flight distance of a paper airplane
Different thickness/types of paper/fabrics affecting the time it takes for it to burn
The effect heat has on the rate of a chemical reaction (by using effervescent tables in water of different temperatures)
Comparing plastic, polystyrene etc. and their ability to keep drinks warm/cool
Colour absorbing heat - different painted tins/containers containing water inside and measuring the temperature of water using a thermometer after time
Do additives such as sugar and salt influence the time for ice cubes to melt? Or does the shape of ice cubes affect the time it takes to melt?
The impact of different salt concentrations on the freezing point of water
Comparing the battery life across different brands of batteries
Viscosity --> How does the viscosity of marbles covered in honey affect the time for it to complete a obstacle course
The different strengthened magnets influencing the number of paper clips it attracts
What temperature conditions are best suited in preventing the loss of carbonation in soda
Investigate/compare the ability of different materials/different paper towels to absorb water/spilled oil
An experiment on bottles containing various amounts of sugar/water temperature to measure the growth of yeast
Does the amount of hours of high intensity cardio exercise per week impact the heart rate reserve of students in anaerobic exercise?
The impact difference music has on concentration through asking participants to memorize and recall a list of words under different musical conditions.