Well, it sounds like you're already off to a good start. Energy can be interchangeable, as you have already described in your example. When you lift the marble up to a certain height above the floor, you are building gravitational energy into the positioning of the marble by doing so. When the marble is released and starts moving on a designed track, the energy from gravitational potential energy (stored energy) starts converting over into kinetic energy (energy of motion). From here, you can have the marble impact a switch, causing a horn to sound, which would be sound energy. That would also be chemical energy, if you are powering the horn with a battery. If the marble turns on a light, that would be light energy. You see, you already had the groundwork for a great idea. All that remains is how you want to design it, then put the plan into action and build it. You should have absolutely no problem demonstrating a minimum of at least three energy transfers.