The HTML 5 canvas object has made Flash obsolete. It is possible to create high quality graphical and animation effects by combining images and line drawing. This basic ‘Cogs’ program illustrates what is possible with elementary JavaScript (though you need to be good at trig!). Circles with serrated edges are drawn on the canvas and the animation feature is used to create the motion.
The students should try to do the same and perhaps design more complex challenges, for example, asking the player to identify the cog that is turning fastest or slowest; changing the speed; allowing the user to drag, place and size cogs, and so on. For more outlandish design ideas, have a look at this Popular Mechanics article, and also a Gear Theory article explaining the correct tooth geometry.