The inquiry based activity I would use and have used in the past with great success is in relation to forces and friction. Working in an all boys school they love their toy cars. They each bring in their favourite toy car from home and we investigate how different materials affect how quickly/slowly the car will drive down a ramp. I provide them with the ramp and materials such as cloth, sandpaper, tin foil, cling film and a smooth piece of plastic. They must predict which surface will be the fastest/slowest for the cars to drive down and why. After investigating this we come back together as a class and compare our results and discuss why they think this is the case. Depending on the classes ability I do sometimes discuss how the different cars they chose can impact the results due to the wheel material, weight of car etc.