The key point is to always find an experiment in which you change one thing and measure one thing (sometimes this is vague when you might measure potential difference and current to get resistance but the research question would still be how does x change resistance?')
Thermoelectric generator (changing area or temperature differential)
Efficiency of transformers with a various factor (changing ratio of coils, efficiency for 1:1 ratios with different numbers of coils)
Resistance of different molarities or different temperatures of solution
Temperature change on gas (tennis ball)
Leidenfrost effect (changing volume of droplet)
Streamlining of wing sections (changing angle of attack, camber, length)
Magnus effect
Lift (rotors including drones or aerofoil, angle, wind speed, CSA of blades)
Drag (adding mass in coffee filters or muffin holders, changing mass on a parachute)
Density of a string/wire and fundamental frequency?
Thickness/internal radius/length of a copper pipe and time for a magnet to drop?
Nichrome wire and frequency (changing temperature, tension)
Doppler effect (changing the angle of the microphone to the pathway of the object when measuring speed)
Solar panels (changing coverage, temperature, wavelength of light received)
Water waves (changing depth, measuring speed)
Rolling cylinder (changing internal radius of cylinder)
Levitating ping pong ball (helpful to have a leaf-blower with a range of settings)
Slip and an inclined plane - rolling a solid cylinder (acceleration change and angle)
Density/viscosity/refractive index of liquid (e.g. glycerol) and temperature
Breaking point of ice
Rolling magnets on an aluminium slope (induced emf, hard to linearise)
Rolling cans with liquids (changing mass or viscosity)
Spring constant change and temperature
Saxon bowl and sinking
Heating magnets and field strength
Lenz’s law (pendulum, differing thickness of copper tubing, number of magnets, initial velocity)
Rotational velocity of rotor systems (e.g. angle of attack, camber, length, fluid properties)
Eddy currents heating/breaking (spinning disc and electromagnet)
Sound transfer through different densities of liquid (battery powered water leakage alarms are useful)
Brewsters angle and change due to temperature change of liquid
Standing waves and end correction, wine glasses
AC generator emf and changing frequency, Helmholz coil and a pendulum oscillating within the loop
Air flow over (or surface area of) an evaporating liquid
Terminal velocity (changing viscosity of fluid, or area with an excellent method...)
Surface tension of liquids (changing salt content, viscosity)
Friction of rubber when heated, pressure of a tyre and friction
Speed of sound in metal rods (change temperature)
Spin on a ping pong ball dropped onto a bat
Poiseuille’s Law (viscosity, flow rate)
Pressure of a ball and impact/collision time
Effect of banking (worked best on rails using a Lego train)
Hydraulic jump (need to 3D print the nozzles)
Angle of a simple pendulum to investigate limitations (expanding the small angle approximation)