I am currently thinking on doing a hand powered fan. This can be very useful to audiences from 5-20 years old. This can be a hand-powered cooler when the climate is very hot during the summer, and also functions as a toy. My audience is 5-20 as children below 5 usually have safety hazards using this, while those above 20 may not find interest or need in the fan. This idea has very specific function and it has a very clear target audience, while clearly showing how energy transfers in this toy. However, the rope in the fan can be very difficult to tie up and its appearance needs to be fixed.
This car could also be another toy that I could make. This targets audiences 5-10, where children in this age is still interested in toys and not yet enter the stage of adolescence. This toy resembles a race car. To function, one must pull it back and release it for it to run. This shows very direct transfer of energy. However, it is very difficult to make and has to be very precise and is easy to break.