I would take a look through the pictures of Solidworks Projects topic on this forum. There are a wide variety of things that people end up using SW to design. Try to make it personally applicable to each participant.
The 3D printer should help alot as it will let you move things from being just lines on the screen to something in hand. I would get that involved early. Early on draw something similar (sphere trapped in a wireframe cube or a planetary gear set or a smiley face drink coaster) and have it printed before the next class to keep them engaged. Even if the gears don't mesh right you can use it as a learning experience about the usefulness of being able to revise the designs.
Car parts (engine or interior), cell phone accessories, jewelery, figurines, video game controller covers, the possibilities are endless. The more ideas you toss out so that each participant sees how they could turn one of their own wishes into reality the better your chances of them having enough interest to follow through.
Most young people today want the immediate gratification so even if you have to take one of their ideas and get a 3D Content Central or GrabCad model to show them what can be done. Or have the major model sharing sites open and ask for some suggestions of things that people think AREN'T relate-able to Solidworks and show them that you can find examples of them that someone has created.