For the past couple of years my students have carried out a design lab based on some solar powered toy cars a lamp and a dimmer switch. I also supply some cheap solar cells so they can measure the pd of the cell without taking it off the car. This pile of apparatus leads to some interesting research questions, many based around the motion of the car. I'm being a bit cagey here as I don't want to give away any ideas to my students. I can see that this sort of exercise would lead to a lot of good investigation topics as the physics required to explain the results is not beyond the average student. The problem with many design labs is that it is very difficult to derive a physical model to explain the outcome. I often use the example of throwing a ball of Plasticine against a wall and measuring the size of the mark it leaves. The size of the mark is certainly related to the speed of the ball but how do you model it mathematically. As we prepare our students for their investigation we need to be thinking of ideas that are interesting but not too difficult. If they don't understand the theory how do they evaluate their results? I am also aware that I mustn't give my students too many potential investigations as labs, don't want to take all the ideas away from them.