My dad and I put together a lot of model cars and rockets when I was a kid. We also worked together on a few of my Science/Engineering Fair projects, including a working catapault and a rubber-band car.

I wish we'd had this little solar-powered marble machine kit back then, because it totally would've been in our wheelhouse. From the company's description:

The circuit takes the light from the solar cell, stores it and releases it in a pulse, making the gear wheel move, which eventually drops the marble down the spiral. In sunlight, the motor will pulse every few seconds, and drop a ball every minute. In indoor light, motor pulses will drop to every few minutes (be patient)! if you are not, then you can use the hand crank and see the marble drop.

Here's a (https://youtube.com/watch?v=f6HKingNstI)

It's not true perpetual motion of course, but pretty neat nonetheless.

The set comes with almost everything you need to assemble the kit. All you need to provide are a Philips #1 screwdriver, a soldering iron, and a little bit of glue. This is a fun and challenging project, and I recommend getting a science-interested kid to help out.

Get the kit for $45 on Amazon.